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).