Basic Subnetting in Computer Networks Explained

An IP subnet is a group of IP addresses. There are two types of IP subnets: default and custom. Default IP subnets are pre-defined and known as IP classes. We create custom subnets as per our requirements and network size. We use subnetting to create custom subnets from default subnets.

Subnetting is a method of dividing a single IP network into small IP networks. Small networks are easier to manage in comparison to large networks.

Let us take an example. There are two cities. The first city uses a flat design for addressing. All addresses directly belong to the city name. They contain only two things: resource name and city name.

The second city uses a hierarchical design for addressing. It divides the city area into blocks and streets. Addresses contain blocks and street numbers or names. Finding an address in the second city is much easier and faster than in the first city.

subnetting advantage example 1

If we relate this example to computer networks, the first city uses a single IP subnet, while the second city divides a single IP subnet into smaller IP subnets.

Let’s take another example. Due to maintenance, there is a scheduled power cut.

subnetting advantage example 2

If the city is divided into sectors, the electricity department may make a local announcement for the affected area instead of making an announcement across the city.

Apart from these two examples, there are many other examples where you can see that small things are easier to manage than big things. The process of breaking a big thing into small things is known as segmentation.

Default IP classes contain a large number of IP addresses in each subnet. A class A IP subnet includes 16777214 IP addresses. A class B IP subnet includes 65534 IP addresses, while a class C IP subnet has 254 IP addresses.

Putting too many hosts in the same network can lead to performance-related issues, such as broadcasts, conflicts, congestion, etc.

Let’s take a simple example.

A company has four divisions; sales, production, development, and management. Each division has 50 users. The company uses a class C IP subnet. Without subnetting, all computers work in a single large network.

subnetting example single network

Computers use broadcast messages to access and provide information in the network. A broadcast message is an announcement message in a computer network that is received by all hosts in the network.

Earlier in this tutorial, we discussed an example of an electric announcement. You can relate that example to this example. In that example, we learned how dividing a city into sectors can reduce announcement areas.

In this example, since all computers belong to the same network, they will receive all broadcast messages regardless the broadcast messages they are receiving are relevant to them or not.

Just like the second city of the first example, we can divide this network into subnets. After this, computers will receive only the broadcasts that belong to them.

Since the company has four divisions, it can divide its network into four subnets. The following figure shows the same network after subnetting.

Subnetting example subnetted network

Subnetting table

The following table lists IP subnets for this network.

Description Network 1 Network 2 Network 3 Network 4
Network address
valid hosts to to to to
Broadcast address

Advantage of Subnetting

  • Subnetting allows us to break a single large network into small networks. Small networks are easy to manage.
  • Subnetting reduces network traffic by allowing only the broadcast traffic that is relevant to the subnet.
  • By reducing unnecessary traffic, subnetting improves the overall performance of the network.
  • By keeping a subnet’s traffic in the subnet, subnetting increases the network's security.
  • Subnetting reduces the requirement for a new IP range for each section.

Disadvantage of Subnetting

  • Different subnets need a router to communicate with each other.
  • Since each subnet uses its own network address and broadcast address, more subnets mean more waste of IP addresses.
  • Subnetting adds complexity to the network. An experienced network administrator is required to manage the subnetted network.

ComputerNetworkingNotes CCNA Study Guide Basic Subnetting in Computer Networks Explained