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’ve ever had the chance to work for a larger company, especially one which has multiple offices, chances are that you’re already acquainted with subnetting in one form or another. In fact, your daily responsibilities, such as communicating with coworkers about a project, were made possible thanks to the security and efficiency provided by subnetting. Employees who aren’t part of the company’s IT department might not always be aware of the huge role played by subnetting in their job, or even how the process works.

In basic terms, subnetting is the creation of a network within a network, often known as a subnetwork – or simply subnet. Also defined as a logical subdivision of an IP network, a subnetwork is more localised and compact than a main network.

Subnetting is most often done for efficiency purposes, allowing network traffic to be streamlined by erasing the need for it to travel through additional routers. This means that the data being transferred can travel to its destination as directly as possible, minimising any potential detours which could slow it down. One way of picturing this is by imagining you’re trying to take the public transport from Camden to the Emirates Stadium in London. In order to do so, you have to travel south on the Northern line to King’s Cross St Pancras, one of the main transport hubs of the city, and change onto the Piccadilly line to go back north again. Subnetting this journey would mean creating an extra line to take passengers from Camden straight to Arsenal by simply going east. This would not only be faster due to a shorter travel distance, but would also help travelers avoid the crowds at one of the UK’s main train stations.

Although companies don’t always have the power to influence public transport infrastructure, they do have a say over the efficiency of their internal networks. Read on to find out more about how subnetting works, and what are some of its advantages.

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.

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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.

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What is a subnet mask?

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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