Introduction to Subnetting

Subnetting allows a network administrator to create a smaller network known as sub-networks or subnets inside a large network by borrowing bits from the Host ID portion of the address. We can also create additional networks using those borrowed bits. It implements and manages an effective IP addressing plan by partitioning a single physical network into more than one smaller logical sub-networks (subnets).

An IPv4 address contains a network portion and a host portion. Both portions of the IP address allow basic network groupings and help routing packets between different networks. The router forwards packets based on the network portion of an IP address and the host portion of the address allows for identification of the destination device. But, with the development of the network, a lot of organizations adding more hosts to their network, the two-level hierarchy is not enough.


Further dividing a network adds another level to the network hierarchy. Division creates three levels: a network, a subnetwork, and a host. Subnetting also allows an organization to add sub-networks without the need to get a new network.

Subnetting is possible due to borrowing the bits from the IP address’s host portion. The borrowing bits assign a number of smaller sub-networks inside the original network. Subnets also decrease network traffic and cover network complexity. Subnetting is necessary when a single network number allocated but more Local Area Network (LAN) is needed.

All IP addresses are divided into different classes and all the classes, such as Class A, B, and C include the subnet mask known as the default subnet mask. We determine the class and number of IP addresses required for a given local network using a subnet mask. The default subnet mask for each class is following

  • A:      or  /8
  • B:   or  /16
  • C:  or /24

We can partition a single Class A, B, or C network into smaller portions using subnetting. It is also possible to subnet a network again and again for more sub-networks. The subnetting a network into a number of subnets provides benefits as follow:

Subnetting Improve network performance and speed

A network is a logical broadcast domain in a computer network. Devices in the same network can reach each other by broadcast at the data link layer. A broadcast domain can be within the same network segment. A single broadcast packet sends out information will reach every device configured into the same network or sub-network. A large network negatively impacts internetwork switching device performance, as well as your network’s overall performance and speed.

Subnetting allows sub-network to maximize their speed effectively. Due to divide the network’s broadcast domains, the network administrator can better control traffic flow, thus increasing network performance. The subnetting also limit the number of devices on the subnet which helps to control the traffic flow and network overhead.


It allowed any organization to subnet their network according to the way they are actually structured in the organization’s physical network. We can also modify the number of sub-networks and number of hosts per sub-network for each organization and they can decide on its own subnet structure and change it as required.

Routing Table size

The larger routing table size reduces the efficiency of the router and smaller routing table speed up all the routing process. We can minimize the size of the routing tables on the internet since additional network which not required will not add to the table.

Simplified Management

The subnetting makes the job of a network administrator a lot easier. Using subnetting administrator can create logical host limits, as opposed to the class full addresses. It makes the host management very easy and whenever an administrator wants to isolate some segments from some other, he can easily do this job.

In classful addressing for more than 254 hosts or devices, we need a Class B network, which can waste thousands of IP addresses. By subnetting, we can select the number of bits according to our host requirement. Subnetted networks are easier to manage and troubleshoot.

Reduce network congestion

We can reduce network congestion through strategic planning of subnets and also can reduce the network’s load and manage traffic efficiently.

Boost network security

Subnetting a large network into multiple networks, we have the ability to isolate the compromised network. With the ability to isolate the compromised networks we can easily prevent further damage to the network and can resolve the problem easily.