Introduction to Subnetting

Subnetting is used to design, implement and manage an effective IP addressing plan. It is used to partition a single physical network into more than one smaller logical sub-networks (subnets). An IPv4 address contains a network portion and a host portion. These two portions of addressing allow for basic network groupings that facilitate in routing packets between different networks. A router forwards packets based on the network portion of an  IP address. When the network is located, the host portion of the address allows for identification of the destination device. But, as networks grow up, with a lot of organizations adding thousands of hosts to their network, the two-level hierarchy is not enough. Subdividing a network adds an another level to the network hierarchy, creating spirit, three levels: a network, a subnetwork, and a host.

Subnetting allows an organization to add sub-networks without the need to get a new network. Subnets are designed by accepting bits from the IP address's host portion and using these bits to assign a number of smaller sub-networks inside the original network. Subnetting helps to decrease the network traffic and conceals network complexity. Subnetting is essential when a single network number has to be allocated over numerous sub-networks of a local area network (LAN).

As we learn in the previous lessons that each IP address consists of a subnet mask. All the class types, such as Class A, B, and C include the subnet mask known as the default subnet mask. The subnet mask is used for determining the type and number of IP addresses required for a given local network. The router is called default gateway. The default subnet mask is as follows:

• Class A: 255.0.0.0      or  /8


• Class B: 255.255.0.0   or  /16


• Class C: 255.255.255.0  or /24

The subnetting allows the network administrator to partition a single Class A, B, or C network into smaller portions. The subnets can be subnetted again into more sub-subnets. Subnetting network into a number of subnets provides the following benefits:

• Improve network performance and speed. Subnetting allows sub-network to maximize their speed and effectiveness. Subnetting also divides network’s broadcast domains, which enabling network administrator to better control traffic flow, thus increasing network performance.


• Better Match to Physical Network Structure. It is allowed any organization to subnet its network that reflect the way they are actually structured in the organization's physical network. 


• Flexibility. The number of sub-networks and number of hosts per sub-network can be modified for each organization and they can decide on its own subnet structure and change it as required. It is also allowing to use two or more LAN technologies together in the same network.


• Routing Table size. Subnets also helpful to minimize the size of the routing tables on the internet since additional network numbers will not be added to the table.


• Simplified Management. Subnetting makes management very easy. When you want to isolate some segments from some other administrator can easily do this.


• Reduce network congestion. Subnetting ensures that traffic destined for a device within a subnet stays in that subnet, which reduces congestion. 


• Boost network security. The network administrator can control the flow of traffic using ACLs, QoS, or route-maps, enabling you to identify threats, close points of entry, and target your responses more easily by using subnetting.