What is subnetting?

Partitioning a single network can help relieve network congestion and increase security

Close-up image of network fibre optic cables

If you have ever been employed at a fairly sizeable organisation or enterprise, such as one that has more than two branches, there’s a strong possibility your workplace is benefiting from something called subnetting.

This process, which is also known as subnetworking, makes it possible to split main networks into two or more strands by breaking down large and unwieldy networks into ones that are more localised and compact.

The process gets its name from the subnetwork, also called a subnet, which is defined as a logical subdivision of an IP network. 

First, a look at IP addresses

It is important to understand IP addresses in order to fully comprehend the process of subnetting. IP addresses are combinations of 32-bit numbers, unique for each device, with values ranging from zero to 4294967295.

They are split into four octets, which is a group of eight bits. The most prevalent visible format of an IP address is created by converting each octet into a decimal, separated by a single dot. This process is capable of producing a total number of 4.3 billion unique IP addresses, almost enough to provide a separate address for over half of the global population.

In an IP address, you’ll find a Network Prefix (or ID) and the Host ID, which can be thought of as two separate fields. These are separated based on one of the five classes of networks in which the IP address has been assigned. The classes are also named after the first five letters in the Latin alphabet, ranging from A to E. In the majority of cases, IP addresses are likely to be placed in the A to C class, with D and E being reserved.

Class A networks use a default subnet mask of 255.0.0.0 and have 0-127 as their first octet. Class B networks use a default subnet mask of 255.255.0.0 and have 128-191 as their first octet. Class C networks use a default subnet mask of 255.255.255.0 and have 192-223 as their first octet.

A Class A, B, or C TCP/IP network can be further divided, or subnetted, by an organisation's IT administrator.

Why use subnetting?

When the IP system was first introduced, it quickly became clear that although it was now far easier to find a specific network, it was also now difficult to send a data packet to the machine you want on that network. This becomes particularly apparent when a network becomes large enough to support an organisation, where network performance becomes more of an issue.

Subnets help to solve this problem by breaking up the network into smaller parts, reducing congestion as a result. Data packets are then able to flow directly to their destination and avoid any individual bottlenecks.

An organisation can use IP subnets to divide larger networks for logical reasons (firewalls, etc), or physical requirements (smaller broadcast domains, etc). In other words, routers use subnets to make routing choices.

Subnetting is also used to improve network security, as the divisions between each subnet allow organisations to enforce access controls - which also helps to contain any security incidents.

What is a subnet mask?

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As with an IP address, a subnet mask comprises four bytes (32 bits) and is written in the same notation as an IP address, typically this is 255.255.255.0. For TCP/IP to work, you need a subnet mask.

The subnet mask complements an IP address and by applying it to the IP address and it determines what subnet an IP address belongs to. An IP address has two components, the network address and the host address. Subnetting further divides the host part of an IP address into a subnet and host address if additional subnetworks are needed. In effect, it masks an IP address and divides the IP address into network address and host address.

What is a default gateway?

When a computer on one network needs to communicate with a computer on another, it uses a router. A router specified on a host, which connects the host's subnet to other networks, is called a default gateway. This passes traffic on one subnet to devices on other subnets. This gateway often connects the local subnet to the internet.

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