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 happen to be employed at a sizable enterprise or organisation, especially one that has more than two branches, there’s a strong possibility that your workplace could be benefitting from a process called subnetting.

Also known as subnetworking, subnetting makes it possible to break down large and unwieldy networks into ones that are more compact and localised, by splitting the main networks into two or more strands.

The process is named after the subnetwork, or subnet, which is defined as a logical subdivision of an IP network.

First, a look at IP addresses

In order to fully understand the process of subnetting, it is important to understand IP addresses: these are combinations of 32-bit numbers, unique for each device, with values ranging from zero to 4294967295. IP addresses are split into four octets, an octet being a group of eight bits. The most commonly seen format of an IP address is created by converting every octet into a decimal, each separated by a single dot. This tactic is capable of generating a total number of 4.3 billion unique IP addresses, which is almost enough to provide a separate IP address for more than half of the global population.

Within an IP, there is a Network Prefix (or ID) and the Host ID, which can be imagined as two separate fields. They are separated based on one of the five classes of networks in which the IP address had been assigned. The classes are named after the first five letters of the latin alphabet, running from A to E. Most 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?

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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