What is subnetting?

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

If you work inside a large organisation or business, particularly one spread across multiple branches, it's highly likely that it deploys a subnet.

A subnetting, or subnetworking, is the process of splitting a single large network into two or more strands. This means that an otherwise mammoth and unwieldy network can be subdivided into smaller, more localised networks.

First, a look at IP addresses

IP addresses are comprised of a 32-bit number with values ranging from zero to 4294967295, split into four octets. In order for it to be read, each octet (a unit consisting of eight bits) is converted to decimal, which are then separated by a single dot, creating the recognisable IP format. Using this method, it's possible to create 4.3 billion unique IP addresses.

IPs also consist of two fields: the Network Prefix (or ID) and the Host ID. The point at which these two are separate depends on the class in which the address is placed. There are five different classes of networks, which run from A to E - the majority of IP addresses are in the A to C class, with D and E being reserved.

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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.

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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 developed, it became clear that although it was now incredibly easy to find a target network, it was relatively difficult to route a data packet to the right machine on that network. This is made even more difficult when networks reach the scale required to support an organisation, where network performance starts to become an issue. 

By breaking the network up into smaller parts, subnets help alleviate this network congestion, however, it also serves to redistribute the network's capacity and allow data packets to flow directly to the target destination without having to squeeze through a single bottleneck.

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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 can also improve network security. With a division between subnets, organisations can control who has access to what. With subnets, security incidents can be better contained.

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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