How to Design a Small Office Network

How to Design a Small Office Network

This blog post is quick tutorial on "How to Design a Small Office Network". To meet user requirements, even small networks require planning and design. Planning ensures that all requirements, cost factors and implementation options are duly considered. Reliability, scalability and availability are important parts of a network design. To support and expand a small network, you need to be familiar with the protocols and network applications running on it. Common network protocols include DNS, Telnet, SMTP, POP, DHCP, HTTP and FTP.
Up to this point in the previous articles of CCNA, we have considered the services that a data network can provide to the human network, we have examined the characteristics of each layer of the OSI model and the operations of the TCP / IP protocols and we observe in detail Ethernet , a universal LAN technology. The next step is to learn how to gather these elements to form a network that works and can be maintained.

1. Typologies of Small Networks

Most companies are small businesses. Therefore, it is expected that most networks are small networks. In small networks, the network design is usually simple. The number and type of devices in the network are considerably reduced compared to a larger network. In general, network topologies for small networks consist of a single router and one or more switches . Small networks can also have wireless access points (possibly built into the router) and IP phones. As for the Internet connection, they usually have a single WAN connection provided by a DSL, cable or Ethernet connection.

The administration of a small network requires many of the same skills needed to manage larger networks. Most of the work focuses on the maintenance and troubleshooting of existing equipment, as well as the protection of network devices and information.

2. SELECTION OF DEVICES FOR SMALL NETWORKS

To meet user requirements, even small networks require planning and design. Planning ensures that all requirements, cost factors and implementation options are duly considered. One of the first design considerations when implementing a small network is the type of intermediary devices that will be used to support the network. For this, several factors must be taken into account:

• Cost : Cost is one of the most important factors when selecting equipment for a small business network. The cost of a switch or router is determined based on its capabilities and characteristics. The capacity of the device includes the number and types of ports available, in addition to the backplane speed. The cost of cable routing necessary to connect each device on the network must also be taken into account.
• Speed ​​and types of ports and interfaces : Choosing the amount and type of ports on a router or switch is a fundamental decision.
• Expandability : Network devices include modular and fixed physical configurations. Fixed configurations have a specific type and amount of ports or interfaces. Modular devices have expansion slots that provide the flexibility to add new modules as requirements increase. Care must be taken to select the appropriate interfaces and modules for specific media, as routers can be used to connect different numbers and types of networks.

Features and services of Operating System

Depending on the version of the operating system, network devices may support certain services and features, for example:

• Security
• QoS
• VoIP
• Layer 3 switching
• NAT
• DHCP

Routers can be expensive depending on the interfaces and features required. Additional modules, such as fiber optics, increase the cost of network devices.

3. IP ADDRESSING FOR SMALL NETWORKS

When implementing a small network, it is necessary to plan the IP addressing space . All hosts within an internetwork must have a unique address.
The following are examples of different types of devices that affect IP design:

• End devices for users
• Servers and peripherals
• Hosts accessed from the Internet
• Intermediary devices
• The planning and registration of the IP addressing scheme help the administrator keep track of the types of devices. For example, if a host address between ranges 50 and 100 is assigned to all servers, it is easy to identify server traffic by IP address.

Note. If the addresses for these resources are not planned and documented, it is not possible to easily control the security and accessibility of the devices. Each of these different types of devices should be assigned to a logical block of addresses within the network's address range.

REDUNDANCY IN SMALL NETWORKS

Another important part of network design is reliability . Even small businesses often rely heavily on the network for their operation. A network failure can have very expensive consequences. To maintain a high degree of reliability, redundancy in network design is required . Redundancy helps eliminate single points of error. There are many ways to obtain redundancy in a network. Redundancy can be obtained by installing duplicate equipment, but it can also be obtained by providing duplicate network links in key areas, as shown in the illustration.

Typically, small networks provide a single point of exit to the Internet through one or more default gateways. With a router in the topology, the only redundancy in terms of Layer 3 routes is obtained using more than one internal Ethernet interface on the router. However, if the router fails, the entire network loses Internet connectivity. For this reason, it may be advisable for small businesses to hire an account with a lower cost option from a second service provider as a backup.

4. Design Considerations for Small Network

Users expect immediate access to their emails and the files they are sharing or updating. To contribute to the assurance of this availability, the network designer must perform the following steps:

1. Step 1 . Provide security to file and mail servers in a centralized location.
2. Step 2 . Protect the location against unauthorized access by implementing logical and physical security measures.
3. Step 3 . Create redundancy in the server farm to ensure that files are not lost if a device fails.
4. Step 4 . Configure redundant paths to servers.

In addition, in modern networks, some form of video or voice over IP is often used to communicate with customers and business partners. This type of converged network is implemented as an integrated solution or as an additional form of raw data overlaid on the IP network. The network administrator must take into account the various types of traffic and their treatment in the design of the network. Routers and switches in a small network must be configured to support real-time traffic, such as voice and video, independently of other data traffic.
Traffic classes can be as specific as the following:

• File Transfer
• Email
• Voice
• Video
• Messenger service
• Transactional

In short, the goal of a good network design, even for a small network, is to increase employee productivity and reduce network downtime.

5. Common applications for small Network

The utility of networks depends on the applications that are in them. Within the application layer there are two forms of software processes or programs that provide access to the network: the network applications and the services of the application layer.

NETWORK APPLICATIONS

Applications are the software programs that are used to communicate through the network. Some end user applications recognize the network, which means that they implement the application layer protocols and can communicate directly with the lower layers of the protocol stack.

• Email clients and Web browsers are examples of these types of applications.

APPLICATION LAYER SERVICES

Other programs may require the assistance of application layer services to use network resources, such as file transfer or managing print queues on the network. Different types of data, whether text, graphics or video, require different network services to ensure that they are properly prepared for processing by the functions found in the lower layers of the OSI model. Each network service or application uses protocols that define the standards and data formats that should be used. Without protocols, the data network would not have a common way to format and address the data. It is necessary to familiarize yourself with the underlying protocols that govern the operation of the different network services to understand their function.

7. COMMON PROTOCOLS OF A SMALL NETWORK

Most of the work of a technician, whether in a small network or a large network, is related in some way to the network protocols. Network protocols support the services and applications used by employees in a small network. Common network protocols include the following:

• DNS : Service that provides the IP address of a Web site or a domain name so that a host can connect to it.
• Telnet : A service that allows administrators to connect to a host from a remote location and control the host as if they were logged in locally.
• IMAP, SMTP, POP (email): Uses the simple mail transfer protocol (SMTP), the post office protocol (POP3) or Internet message access protocol (IMAP).
• DHCP : Service that assigns the IP address, subnet mask, default gateway and other information to clients.
• HTTP : Used to transfer information between Web clients and Web servers.
• FTP : Service that allows the download and upload of files between a client and a server.

These network protocols make up the fundamental toolset of network professionals. Each of these network protocols defines the following:

• Processes at any end of a communication session.
• Types of messages
• Message syntax.
• Meaning of the information fields.
• How messages are sent and the expected response.
• Interaction with the next lower layer.

Many companies established a policy of using secure versions of these protocols, whenever possible. These protocols are HTTPS, SFTP and SSH.

8. REAL-TIME APPLICATIONS FOR SMALL NETWORKS

In addition to the common network protocols described above, modern companies, including small ones, often use real-time applications to communicate with customers and partners. While it is possible that a small company cannot justify the cost of a Cisco Telepresence solution for businesses, there are other real-time applications that are accessible and justifiable for small businesses. The network administrator must ensure that the appropriate equipment is installed on the network and that the network devices are configured to ensure delivery according to priorities.

• Infrastructure:  To support proposed and existing real-time applications, the infrastructure must adapt to the characteristics of each type of traffic. It must be determined whether existing switches and wiring can support the traffic that will be added to the network.
• VoIP : VoIP uses routers with voice capabilities. These routers convert the analog voice of traditional telephone signals into IP packets. After that, the router sends these packets between the corresponding locations. VoIP is much cheaper than an integrated IP telephony solution.
• IP Telephony : In IP telephony, the IP telephone itself performs voice to IP conversion. In networks with integrated IP telephony solution, routers with voice capabilities are not required. IP phones use a dedicated server for call control and signaling.
• Real-time applications : To transport streaming media effectively, the network must be able to support applications that require time-dependent delivery. The Real-Time Transport Protocol (RTP) and the Real-Time Transport Control Protocol (RTCP) support this requirement.

9. SCALING OF SMALL NETWORKS

Growth is a natural process for many small businesses, and their networks must grow accordingly. Ideally, the network administrator has sufficient time to make intelligent decisions about the growth of the network in relation to the growth of the company.
To scale a network, several elements are required:

• Network documentation : physical and logical topology.
• Device inventory : list of devices that use or make up the network.
• Budget : detailed IT budget, including equipment acquisition budget for the fiscal year.
• Traffic analysis : protocols, applications, services and their respective traffic requirements must be registered.

These elements are used to support the decision making that accompanies the scaling of a small network.

10. EVOLUTION OF THE PROTOCOL REQUIREMENTS

Network administrators should be aware of how network use changes. As shown in the illustration, small network administrators have the ability to obtain “snapshots” of IT in person from the use of applications by employees for a considerable portion of the workforce over time.
Generally, these snapshots include the following information:

• OS and OS version
• Non-Network Applications
• Network applications
• CPU usage
• Unit utilization
• RAM utilization

The registration of snapshots of employees in a small network during a given period is very useful for informing the network administrator about the evolution of protocol requirements and related traffic flows.

For example, some employees may use external resources, such as social media, to better position a company in terms of marketing. When these employees started working for the company, they may not have given so much importance to Internet-based advertising. This change in resource utilization may require the network administrator to change the allocation of network resources accordingly.

Application Layer Protocols and Services

Application Layer Protocols and Services

In this article we will learn about application layer protocols and services . We will see how each of them works. Following is the list of most popular protocols of this layers:

TCP / IP application layer protocols table

Protocol	Description
File Transfer Protocol (FTP)	It is used for interactive transfer of connection-oriented files, requires ACK. TCP 20, 21
Trivial File Transfer Protocol (TFTP)	It is used for simple and offline file transfer. UDP 69
Simple Mail Transfer Protocol (SMTP)	It is used to transfer (send) messages and email attachments. TCP 25
Post Office Protocol (POP)	It is used by email clients to retrieve and download email from a remote server. TCP 110
Internet Message Access Protocol (IMAP)	Another protocol for retrieving email stored on a server. It keeps the messages on the server. TCP 143
Domain Name Service Protocol (DNS)	It is used to resolve Internet names in IP addresses. TCP, UDP 53
Telnet: terminal emulation protocol	It is used to provide remote access to servers and network devices. TPC 23
Bootstrap Protocol (BOOTP)	a precursor to the DHCP protocol. Network protocol used to obtain IP address information during startup. UDP 68 CUSTOMER, UDP 67 SERVER
Dynamic Host Configuration Protocol (DHCP)	It is used to assign an IP address, a subnet mask, a default gateway and a DNS server to a host. UDP 68 CUSTOMER, UDP 67 SERVER
Hypertext Transfer Protocol (HTTP)	It is used to transfer files that make up the Web pages of the World Wide Web. TCP 80, 8080
Safe Hypertext Transfer Protocol (HTTPS)	The browser uses encryption to protect HTTP communications, authenticates the website. TCP, UDP 443

HTTP and HTML

When you type a web address or a uniform resource locator (URL) in a web browser, the browser establishes a connection to the web service that runs on the server using the HTTP protocol.

URL example: https://ccnacompletecourse.blogspot.com

First, the browser interprets the three parts of the URL:

• https (the protocol or scheme)
• ccnacompletecourse.blogspot.com (the name of the server)

Through the requirements of the HTTP protocol, the browser sends a GET request to the server and requests the file.

HTTP AND HTTPS

HTTP is a request / response protocol. When a client, usually a web browser, sends a request to a web server, HTTP specifies the types of messages that are used for that communication. The three types of common messages are GET, POST and PUT.

1. GET is a request for data from the client. A web browser sends the GET message to the web server to request the HTML pages
2. POST and PUT are used to send messages that upload data to the Web server.

Although HTTP is extremely flexible, it is not a secure protocol. In summary:

• HTTP is not secure. The messages can be intercepted.
• HTTPS uses authentication and encryption to secure data.

EMAIL PROTOCOLS 

• Email is a storage and shipping method used to send, store and retrieve electronic messages.
• Email messages are stored on mail servers.
• Email clients communicate with email servers to send and receive email messages.
• Mail servers communicate with other mail servers to transport messages from one domain to another.
• Email depends on three separate protocols to work: SMTP, POP and IMAP.

SMTP OPERATION

SMTP message formats need a header and a message body. While the body of the message can contain the desired amount of text, the header must have a properly formatted recipient email address and a sender address.

When a client sends email, the client's SMTP process connects to a server's SMTP process on the well-known port 25 . After the connection is established, the client tries to send the email to the server through it. Once the server receives the message, it locates it in a local account (if the recipient is local) or forwards it to another mail server for delivery. The destination mail server may be disconnected or busy. In these cases, the SMTP queues the messages to send them later. The server periodically checks the queue for messages and tries to send them again. If the message has not yet been delivered after a predetermined expiration time, it will be returned to the sender as impossible to deliver.

POP OPERATION

POP is used by an application to retrieve email from a mail server. With POP, mail is downloaded from the server to the client and then deleted on the server. POP works this way, by default. POP3 is desirable for ISPs, as it lightens their responsibility for handling large amounts of storage for their email servers. The server starts the POP service by passively listening to client connection requests on TCP port 110 . When a client wishes to use the service, it sends a request to establish a TCP connection with the server. Once the connection is established, the POP server sends a greeting. Next, the client and the POP server exchange commands and responses until the connection is closed or canceled.

IMAP OPERATION

IMAP is another protocol that describes a method to retrieve email messages. Unlike POP, when the user connects to a server with IMAP capability, copies of the messages are downloaded to the client application . The original messages are kept on the server until they are manually deleted. Users see copies of the messages in their email client software.

Users can create a hierarchy of files on the server to organize and save mail. This file structure is also duplicated in the email client. When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.

DYNAMIC HOST CONFIGURATION PROTOCOL (DHCP)

• Dynamic Host Configuration Protocol (DHCP) allows a host to obtain an IP address dynamically.
• The DHCP server is contacted and the address is requested; it chooses the address of a configured address range called "pool" and is granted to the host for a set period.
• The addresses are returned to the pool for reuse when they are no longer in use.
• DHCP is used for general-purpose hosts, such as devices for end users; Static addressing is used for network devices such as gateways, switches, servers and printers.

DHCP OPERATION

• The client transmits a DHCPDISCOVER message whose destination is broadcast (L2 FF-FF-FF-FF-FF-FF and L3 255.255.255.255), to identify any DHCP server available on the network.
• This request reaches all PCs on the Network but only the DHCP server responds to the host with a DHCPOFFER message.
• The client sends a DHCPREQUEST message to the server that it wishes to use (in the case of several offers).
• A client can also request an address previously assigned by the server.
• The server returns a DHCPACK message to confirm that the grant has ended.

FILE TRANSFER PROTOCOL (FTP)

FTP is another application layer protocol that is commonly used:

• An FTP client is an application that runs on a client computer and is used to insert and extract data on an FTP server.
• FTP requires two connections between the client and the server: one for commands and responses (TCP port 21), and another for file transfer (TCP port 20) itself.
• The client can download (extract) data from the server or upload (insert) data to the server.

SERVER MESSAGE BLOCK (SMB)

• SMB is a client / server file exchange protocol.
• All SMB messages share the same format.
• Printing and file sharing services from SMB became the mainstay of Windows networks.
• Microsoft products now support TCP / IP protocols to directly support the sharing of SMB resources.
• Mac, LINUX and UNIX operating systems have their own SMB implementation.
• After establishing the connection, the client user can access the resources on the server as if the resource were local to the client host.

SMB messages can:

• Start, authenticate and end sessions
• Control access to files and printers
• Authorize an application to send or receive messages to or from another device

DOMAIN NAME SERVICE (DNS)

Domain names were created to convert numerical addresses into a simple and recognizable name. Of course, it is simpler to work with domain names than with IP numbers; In addition, if the IP address is changed, the end user does not need to know about the change.

DNS MESSAGE FORMAT

The DNS server stores different types of resource records used to resolve names. These records contain the name, address and type of record. The common DNS records are:

• A : an IPv4 terminal address
• NS : an authoritative name server
• AAAA : a terminal IPv6 address
• MX : a mail exchange record
• DNS servers search their own records first, and relay the client's request to other servers if they cannot resolve it. Then the answer is forwarded to the client.

The ipconfig / displaydns command in Windows shows all DNS stored in the cache.

 DNS HIERARCHY

The DNS protocol uses a hierarchical system to create a database that provides name resolution. The hierarchy is similar to an inverted tree with the root at the top and the branches below

Top-level domains represent the type of domain or country of origin.

Examples of top-level domains are .com, .org, .au and .co. In the DNS hierarchy, top-level domain servers (.com, .gov, .edu) contain records of second-level domain servers. The second level domain name server (cisco.com, etc.) is an authoritative DNS server, since it keeps all the records of the cisco company.

What is Application Layer in OSI Model

What is Application Layer in OSI Model?

This article is a complete brief on What is Application Layer in OSI Model. We will explain the way in which the functions of the application layer, session layer and presentation layer operate together to provide network services to end-user applications.

WHAT IS THE APPLICATION LAYER?

The application layer is closest to the end user. As shown in the image, it is the layer that provides the interface between the applications used for communication and the underlying network in which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. The top three layers of the OSI model (application, presentation and session) define functions of the single TCP / IP application layer.

PRESENTATION LAYER

The Presentation Layer has three main functions:

• The Format : Format the data of the source device, or present it, in a compatible way for the destination device to receive it
• Compression : Compress the data so that it can be decompressed by the target device.
• Encryption : Encrypt data for transmission and decrypt upon receipt. Common video standards include QuickTime and the Movie Expert Group (MPEG) .The common graphic image formats are: GIF, JPEG and PNG.

SESSION LAYER

As the name implies, the functions of the session layer create and maintain dialogues between the source and destination applications and also manage the exchange of information to initiate dialogues, to keep them active and to restart sessions. The so-called layer five of the OSI model, defines the connection of a user computer to a network server and from one computer to another in a network with peer-to-peer configuration.

TCP / IP APPLICATION LAYER PROTOCOLS

There are many application layer protocols, and they are constantly developing. Some of the best known application layer protocols include the hypertext transfer protocol (HTTP), the file transfer protocol (FTP), the trivial file transfer protocol (TFTP), the Internet message access protocol (IMAP) and the Domain Name System (DNS) protocol.
In summary:

• Specify the control information and format necessary for common Internet functions.
• They must be implemented on both the source and destination devices.
• The application layer protocols that are implemented on the source and destination host must be compatible to allow communication.

INTERACTION OF APPLICATION PROTOCOLS WITH APPLICATIONS FOR END USERS

Finally, we see how application protocols interact with applications for end users:

CLIENT-SERVER MODEL

• The device that requests information is called "client" and the device that responds to the request is called "server".
• Client and server processes are considered part of the application layer.
• The content of the data exchange will depend on the application in use.
• Email is an example of a client and server interaction.

POINT-TO-POINT NETWORKS (PEER NETWORK - P2P)

• Two or more computers can be connected to a P2P network to share resources without having a dedicated server.
• Any connected terminal (known as "point") can function as a server and as a client.
• Client and server functions are set by request.

A P2P application allows a device to function as a client and as a server within the same communication. In this model, each client is a server and each server is a client. P2P applications require that each terminal provide a user interface and run a background service.

TCP Vs UDP | Explanation and Comparison TCP UDP port Numbers

TCP Vs UDP | Explanation and Comparison

This article is about TCP Vs UDP protocol, Explanation and its difference. Also the reason why the TCP protocol is considered more reliable than UDP protocol. We will discus both protocol in details and in the end we will compare TCP and UDP.

TCP Protocol

The function of the TCP transport protocol is similar to sending packets that are tracked from source to destination. If a shipping order is divided into several packages, the customer can check the delivery order online.
There are three basic operations that enable reliability with TCP:

1. The numbering and tracking of data segments transmitted to a specific host from a given application
2. The acknowledgment of data receipt
3. The retransmission of any data without acknowledgment after a certain period

• TCP FEATURES

• Connection oriented : create a session between the source and destination.
• Reliable delivery : retransmit lost or damaged data.
• Reconstruction of ordered data : numbering and sequencing of segments to rearm data in the correct order.
• Flow control : regulates the amount of data that is transmitted since network hosts have limited resources, such as memory or processing capacity.
• Protocol with status : tracks the session.

• TCP HEADER

TCP is a protocol with status information, that is, it tracks the status of the communication session. TCP adds 20 bytes of overhead to the segment header

• Source port (16 bits) and destination port (16 bits) : used to identify the application.
• Sequence number (32 bits) : used to rearm data.
• Recognition number (32 bits) : indicates the data received.
• Header length (4 bits) : known as “data offset”. Indicates the length of the TCP segment header.
• Reserved (6 bits) : this field is reserved for the future.
• Control bits (6 bits) : includes bit codes, or markers, that indicate the purpose and function of the TCP segment.
• Window size (16 bits) : indicates the number of bytes that can be accepted at a time.
• Checksum (16 bits) : used to verify errors in the header and segment data.
• Urgent (16 bits) : indicates if the information is urgent.

• TCP COMMUNICATION PROCESS
• Each application process that runs on the server uses a port number.
• An individual server cannot have two services assigned to the same port number within the same transport layer service.
• An active server application assigned to a specific port is considered open.
• There may be many ports open simultaneously on one server, one for each active server application.

A TCP connection is established in three steps:

• The source client requests a client-to-server communication session with the server.
• The server acknowledges receipt of the client-to-server communication session and requests a server-to-client communication session.
• The source client acknowledges receipt of the server-to-client communication session.

The TCP FIN indicator (completion control marker) is set in the segment header to terminate a TCP connection.

• When the client has no more data to send in the transmission, it sends a segment with the FIN indicator set.
• The server sends an ACK to acknowledge the FIN to receive the client-to-server session.
• The server sends an END to the client to end the session from server to client.
• The client responds with an ACK to acknowledge the FIN from the server.
• Once all segments are recognized, the session is closed.

• TCP THREE-WAY LINK PROTOCOL

To establish the connection, the hosts make a three-way link. The three way link:

• It states that the target device is present in the network.
• Verify that the destination device has an active service and accept requests on the destination port number that the source client wishes to use for the session.
• Inform the destination device that the source client tries to establish 
• a communication session on that port number

TCP control bit fields table

Countryside	Description
URG	important urgent pointer field
ACK	important field of constitution
RST	restore connection
SYN	synchronize sequence numbers
FINISH	no more issuer data

The three steps during a three-way TCP link protocol are SYN, SYN-ACK and ACK.

• TCP RELIABILITY
• TCP segments use sequence numbers to reassemble the segments in the original order.
• During the configuration of the TCP session, a random initial sequence number (ISN) is chosen. Then, the ISN increases with the number of bytes transmitted.
• The data is delivered to the application layer only when it has been received and completely reassembled.
• TCP FLOW CONTROL

TCP also offers flow control mechanisms, the amount of data that the destination can receive and process with reliability. The flow control allows to maintain the reliability of the TCP transmission by adjusting the speed of the data flow between the source and the destination for a given session. To achieve this, the TCP header includes a 16-bit field called " window size ".

• Window size : amount of data that an origin can transmit before receiving an acknowledgment. It determines the amount of data that the target device of a TCP session can accept and process at a time.
• The acknowledgment of receipt . It is the next byte that the destination expects to receive.
• Finally, network congestion usually results in packet discarding.

UDP PROTOCOL

UDP provides the basic functions to distribute data segments among the appropriate applications, with very little overhead and data checking. While TCP's reliability functions provide stronger communication between applications, they also represent an additional overhead and may cause transmission delays. There is a trade-off between the value of reliability and the burden that it implies for network resources. Adding overhead to ensure reliability for some applications could reduce the utility to the application and even be harmful.

UDP is ideal for applications that do not require reliability.

• UDP FEATURES
• Without connection
• Less transmission delays.
• There is no reconstruction of ordered data (performed by the application)
• Unreliable Delivery
• No flow control
• Stateless protocol, neither the client nor the server is obliged to track the status of the communication session.
• UDP HEADER

UDP is a protocol without status information, so if reliability is desired it must be handled by the application layer. Portions of communication in UDP are called datagrams and UDP adds only 8 bytes of overhead.

• UDP COMMUNICATION PROCESS

UDP datagram reset

• UDP simply reassembles the data in the order in which they were received and sends them to the application.
• If necessary, the application must identify the correct sequence.

UDP server processes and requests

• UDP-based server applications are assigned known or registered port numbers.
• Requests received at a specific port are forwarded to the appropriate application based on the port numbers.

UDP client processes

• The UDP client process dynamically selects a port number and uses it as the source port.
• Typically, the destination port is the known or registered port number that is assigned to the server process.
• The same pair of source or destination ports is used in the header of all datagrams used in the transaction.
• When returning data from the server to the client, the source and destination port numbers are reversed in the datagram header.

SEVERAL CONVERSATIONS SEPARATELY

The transport layer separates and manages several communications with different transport requirements. 

• Different applications send and receive data on the network simultaneously.
• TCP and UDP manage these different simultaneous conversations through header fields that can uniquely identify these applications.

These unique identifiers are port numbers.

he source port number is associated with the application that originates the communication on the local host and the destination port is used to identify an application on the server (destination) -> remote host.

UDP and TCP Port Number

Port Number	Protocol	Application	Acronym
20	TCP	File Transfer Protocol (data)	FTP
21	TCP	File Transfer Protocol (control)	FTP
22	TCP	Safe Shell	SSH
23	TCP	Telnet	-
25	TCP	Simple File Transfer Protocol	SMTP
53	TCP, UDP	Domain Name Service	DNS
67	UDP	Dynamic host configuration protocol (server)	DHCP
68	UDP	Dynamic host configuration protocol (client)	DHCP
69	UDP	Trivial File Transfer Protocol	TFTP
80	TCP	Hypertext Transfer Protocol	HTTP
110	TCP	Post Office Protocol version 3 (Post Office Protocol version 3)	POP3
143	TCP	Internet Message Access Protocol	IMAP
161	UDP	Simple Network Management Protocol	SNMP
443	TCP	Secure hypertext transfer protocol	HTTPS

A server can offer more than one service simultaneously, for example, web services (80) and FTP connection (21)

PORT NUMBER GROUPS

The Internet Assigned Numbers Authority (IANA) is the regulatory body responsible for assigning different addressing standards, including port numbers. There are different types of port numbers:

• Well-known ports : These numbers are reserved for services and applications. (For example: web browsers, email clients)
• Registered ports : These port numbers are assigned to an entity that requests them for use with specific processes or applications. For example, Cisco port 1985 for its routing process (HSRP).
• Dynamic or private ports : (or ephemeral) The client OS dynamically assigns them when a connection to a service is initiated.

Port Number Groups Table

Port Number Range	Port Group
Between 0 and 1023	Well-known ports
From 1024 to 49151	Registered Ports
From 49152 to 65535	Dynamic or private ports

THE NETSTAT COMMAND

It is used to inspect TCP connections that are open (active) and running on the network host. The netstat command will attempt to resolve IP addresses in domain names and port numbers in known applications. The -n option can be used to display IP addresses and port numbers in their numerical format.

TCP OR UDP

Which transport layer protocol is correct for the appropriate application? For some applications, the segments must arrive in a very specific sequence so that they can be processed correctly. With other applications, the data is considered useful once all are received in full. In both cases, TCP is used as the transport protocol

• TCP is better for databases, web browsers, email clients, etc. (Telnet, FTP, SMTP, HTTP)

In other cases, an application can tolerate some data loss during transmission over the network, but transmission delays are not allowed. UDP is the best option for these applications, since less network overhead is required.

• UDP is better for live streaming audio or video, VoIP, etc. (TFTP, IPTV, DHCP, DNS SNMP, VoIP)

What is the Transport Layer Responsibilities & Function

What is the Transport Layer Responsibilities & Function

In this we will describe What is the Transport Layer Responsibilities & Function in the administration of data transport in end-to-end communication.
What is the Transport Layer ?
The transport layer, or layer four of the OSI model, manages the flow of information from one network node to another. Its main function is to establish a temporary communication session between two applications and to transmit data between them. TCP / IP uses two protocols to achieve this:

• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)

Responsibilities OF the Transport Layer

The tasks of the transport layer are as follows:

• Track individual conversations between applications on the source and destination hosts : In the transport layer, each set of data that flows between a source and a destination application is known as a “conversation”. A host can have several applications that communicate over the network simultaneously
• Segment data for administration and reassemble segments at the destination : Transport layer protocols have services that segment application data into blocks of an appropriate size. These services include the necessary encapsulation in each piece of data.
• Identify applications : The transport layer assigns an identifier to each application, called a port number. All software processes that require access to the network are assigned a unique port number for that host.

Multiplexing Conversation

Sending some types of data (such as a video stream) through a network, such as a full flow of communication, can consume all available bandwidth. This will prevent other communications from occurring at the same time. It could also hinder error recovery and re-transmission of damaged data.

How to solve it? through the segmentation of data into smaller parts, allowing multiple communications of different users to be interlaced (multiplexed) on the same network.

DATA SEGMENTATION

It allows multiple different communications from several different users to be interlaced (multiplexed) on the same network simultaneously.

It provides the means to send and receive data during the execution of several applications.

A header is added to each segment to identify it.

RELIABILITY OF THE TRANSPORT LAYER

While the IP deals only with the structure, addressing and packet routing. Transport protocols specify the way in which messages are transferred between hosts . TCP / IP provides two transport layer protocols: the transmission control protocol (TCP) and the user datagram protocol (UDP). IP uses these transport protocols to enable communication and data transfer between hosts.

TCP is considered a reliable and complete transport layer protocol, as it ensures that all data reaches the destination. However, this requires additional fields in the TCP header that increase the packet size and also the delay. In contrast, UDP is a simpler transport layer protocol, although it does not provide reliability. Therefore, it has fewer fields and is faster than TCP. We will analyze it later.

Network IP Address Planning for IPv6 & IPv4

Network IP Address Planning

The Network IP Address Planning is required for the allocation of the address space to design a computer network. Network subnet planning requires an analysis of both the organization's network usage needs and how the subnets will be structured. The starting point is to carry out a study of the network requirements.

IP Address Planning

The IP Address Planning includes to determining the following:

• The needs of each subnet in terms of size
• Number of hosts per subnet
• The way host addresses will be assigned
• Which are the hosts that will require static IPv4 addresses and which ones can use DHCP

The network administrator decides the range of private IPv4 addresses used in a LAN, and should carefully consider it to ensure that there are enough host addresses available for known hosts so far and for future expansions. Remember that the ranges of private IPv4 addresses are as follows:

• 10.0.0.0 to 10.255.255.255, with subnet mask 255.0.0.0 or / 8
• 172.16.0.0 to 172.31.255.255, with subnet mask 255.240.0.0 o / 12
• 192.168.0.0 to 192.168.255.255, with subnet mask 255.255.0.0 or / 16

Learn about IP subnetting

Main Consideration while IP Address Planning

The assignment of network addresses should be planned and recorded for the following purposes:

• Avoid duplication of addresses : Without proper planning and registration, one address could be assigned to more than one host, which would cause access problems for both hosts.
• Provide and control access : The Layer 3 address assigned to a server can be used to control access to that server.
• Control security and performance : To easily find problematic network devices.

 ASSIGNING ADDRESSES TO DEVICES

Within a network, there are different types of devices that require addresses, some common devices are:

• End user customers : Most networks assign addresses dynamically with the Dynamic Host Configuration Protocol (DHCP).
• Servers and peripherals (eg printers): they must have a predictable static IP address.
• Servers that can be accessed via the Internet : In most cases, these servers are assigned private addresses internally, and the router or firewall on the perimeter of the network must be configured to translate the internal address to a public address .
• Intermediary devices : Because it is necessary to know how to communicate with intermediary devices, these must have assigned predictable and static addresses.
• Gateway : Normally, the router interface uses the lowest or highest address in the network.

Example network addressing plan: 192.168.1.0/24

USE	FIRST	LAST
Host devices	.one	.229
Servers	.230	.239
Printers	.240	.249
Intermediary Devices	.250	.253
Gateway	.254

DESIGN CONSIDERATIONS FOR IPV6

IPv6 subnetting requires a different approach than IPv4 subnetting. The same reasons for subnetting the IPv4 address allocation space to manage network traffic also apply to IPv6. However, due to the large number of IPv6 addresses, it is no longer necessary to worry about conserving addresses. In IPv6 subnetting, address space conservation is not a problem to consider. The subnet ID includes more subnets than enough. IPv6 subnetting has to do with creating an addressing hierarchy based on the number of subnets needed.

Remember that there are two types of assignable IPv6 addresses: a link-local IPv6 address is never subdivided, since it only exists on the local link; however, a global unicast IPv6 address can be divided.

GLOBAL UNICAST IPV6 ADDRESS

The global unicast IPv6 address usually consists of a global routing prefix / 48, a 16-bit subnet ID and a 64-bit interface ID.

 SUBNETTING BY SUBNET ID

The 16-bit subnet ID section of the global unicast IPv6 address can be used to create internal subnets. The subnet ID provides more subnets than necessary and supports more hosts than may be needed for a subnet. For example, the 16-bit section allows you to do the following:

• Create up to 65,536 subnets / 64. This does not include the ability to borrow bits from the address interface ID.
• Support up to 18 trillion host IPv6 addresses per subnet (e.g., 18,000,000,000,000,000,000).

 IPV6 SUBNET ASSIGNMENT

With the ability to choose from more than 65,000 subnets, the task of the network administrator becomes the task of designing a logical scheme to address the network. As shown in image 1, the topology used as an example will require subnets for each LAN, as well as for the WAN link between R1 and R2. With IPv6 the subnet of the WAN link is not further divided into subnets. Although this may cause the "waste" of addresses, that is not a cause for concern when using IPv6.

As shown in image 2, for this example the assignment of five IPv6 subnets is used, with the subnet ID field from 0001 to 0005. Each subnet / 64 will provide more addresses than will ever be needed.

Finally, the configuration for Router 1 (R1) would be as follows:

R1 (config) # interface gigabitethemet 0/0 

R1 (config-if) # ipv6 address 2001: db8: acad: 1 :: 1/64 

R1 (config-íf) # exit 

R1 (config) # interface gigabitethernet 0/1 

R1 (config-if) # ipv6 address 2001: db8: acad: 2 :: 1/64 

R1 (config-if) # exit 

R1 (config) # interface serial 0/0/0 

R1 (config-if) # ipv6 address 2001 : db8: acad: 3 :: 1/64 

R1 (config-íf) # end 

R1 #

Subnetting How to Divide a Network into Subnets

Subnetting How to Divide a Network into Subnets

In this article we will learn about ip addressing and subnetting & How to Divide a Network into Subnets. The process of segmenting a network by dividing it into several smaller network spaces is called "subnetting." We explain the reasons for subnetting and how to do it.

Network Segmentation

It is necessary to segment large networks into smaller subnets, creating smaller groups of devices and services for the following purposes

• Since each broadcast domain connects to a different router interface, each domain needs its own network address space.
• The process of dividing a range of addresses into smaller address spaces is called subnetting .
• Network administrators can group devices into subnets that are defined by location, organizational unit or device type.
• Control traffic by containing broadcast traffic within the subnet.
• Reduce overall network traffic and improve network performance.

Communication Between the Different Subnets

• A router is needed so that devices on different networks and subnets can communicate.
• Each router interface must have an IPv4 host address that belongs to the network or subnet to which the router interface is connected. Each router interface generates a broadcast domain.
• Devices on a network and a subnet use the router interface connected to your LAN as the default gateway.

LIMITS OF THE OCTET

Networks are more easily subdivided into the limit of the octet of / 8/16 or / 24. Also known as IPv4 Classes , it uses octet limits to separate host networks.

IPv4 subnets are created using one or more of the host bits as network bits. This is done by extending the subnet mask to borrow some of the bits from the host portion of the address to create additional network bits. The more host bits borrowed, the greater the number of subnets that can be defined.

Example : Assume that a company chose as its internal network address the private address 10.0.0.0/8. Said network address can connect 16,777,114 hosts in a broadcast domain. Of course, this is not ideal.

The company can continue subnetting the address 10.0.0.0/8 in the limit of the octet of / 16, as shown in the image. This would give you the ability to define up to 256 subnets (e.g., 10.0.0.0/16 to 10.255.0.0/16), and each subnet would be able to connect 65,534 hosts. Notice how the first two octets identify the network portion of the address and the last two correspond to the host IP addresses.

Subnetting Classes

In the previous example, host bits of the network prefixes / 8 (as common as / 16 and / 24) were borrowed. However, subnets can borrow bits from any host bit position to create other masks, allowing much more flexibility.

As shown in the previous image:

/ 25: Borrowing 1 bit of the fourth octet creates 2 subnets that support 126 hosts each.
/ 26: Borrow 2 bits create 4 subnets that support 62 hosts each.
/ 27: Borrow 3 bits create 8 subnets that support 30 hosts each.
/ 28: Borrowed 4 bits creates 16 subnets that each support 14 hosts.
/ 29: Borrowed 5 bits creates 32 subnets that support, each, 6 hosts.
/ 30: Borrow 6 bits create 64 subnets that support, each, 2 hosts.

SUBNETTING BASED ON THE NEED FOR NETWORKS

There are two factors that must be taken into account when planning subnets:

• Number of subnets required
• Number of host addresses required

EXAMPLE BASED ON NETWORK REQUIREMENTS

Network administrators must design a network addressing scheme that supports the maximum number of hosts for each network and the number of subnets. The addressing scheme must allow the growth of both the number of host addresses per subnet and the total number of subnets.

Example based on network requirements

What we can describe from the topology:

The topology for branches consists of 5 LAN segments and 4 network interconnection connections between routers. Therefore, 9 subnets are required. The largest subnet requires 40 hosts.
The network address 172.16.0.0/22 ​​has 10 host bits. Because the largest subnet requires 40 hosts, a minimum of 6 host bits must be borrowed to provide the address of the 40 hosts. This is determined by the formula 2 ^ 6 - 2 = 62 hosts.
The formula for determining subnets gives a result of 16 subnets: 2 ^ 4 = 16. Since the interconnection of networks that was used as an example requires 9 subnets , this meets the requirement and allows some additional growth.
Therefore, the first 4 host bits can be used to assign subnets, as shown in the following image. When 4 bits are borrowed, the new prefix length is / 26, with subnet mask 255.255.255.192.

Finally, subnets can be assigned to LAN segments and router to router connections.

WASTE OF ADDRESSES IN SUBNETTING

In the traditional subnetting we see that the same number of addresses is assigned to each subnet. We also realize that subnets that require fewer addresses have unused (wasted) addresses. For example, WAN links only need two addresses.

This is where the importance of the variable length subnet mask (VLSM) , or subnet subdivision, allows for more efficient use of addresses (prevents addresses from being wasted).