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.
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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