Subnetting Based on Network Requirement

Sometimes a number of sub-networks are most important than on the number of host addresses per sub-network. For example, an organization wants to separate their network traffic based on internal structure or department setup. In this case, the number of subnets is most important in determining how many bits to borrow. Accommodate the maximum number of the host including the required number of subnets is best practice in networking. The addressing scheme must allow for expansion in the number of host addresses per subnet and the total number of subnets. In this lesson, I am going to discuss the subnetting based on network requirement.

We borrow bits from the host portion into the network portion to create subnets. The formula to calculate the subnet in case of borrowing bits are 2ⁿ (where n is the number of bits borrowed). The important thing is to balance the number of hosts required and the number of hosts needed for the largest subnet. The more bits borrowed to create additional subnets means fewer hosts per subnet.

Examples Subnetting Based on Network Requirement

In this example, an organization has allocated a network address of 115.255.240.0/22. You can see that the network prefix is 22, which means there are 8 bits available in the host portion. So the number of hosts is 2¹⁰-2=1022. Now the organization required to isolate the traffic of all departments from each other. So, we will do the subnetting based on network requirement. The topology for the departments are shown in the figure below, consists of 5 LAN segments and one router segment. So 6 subnets required for this topology. The largest subnet requires 120 hosts and the smallest segment requires 30 hosts.

The 115.255.240.0/22 network address has 10 host bits. The largest subnet requires 120 hosts, which required 7 host bits to provide 126 usable IP addresses for hosts. The formula for determining a host is 2⁷ – 2 = 126 hosts. So if we required 7 bits for host portion then we can borrow 3 bits from host portion for subnetting.

We can determine the subnet using the formula: 2³ = 8.  Therefore, the first 3 bits of the host portion can be used to assign subnets, as shown in Figure below. When 3 bits borrowed, the new prefix length is /25 with a subnet mask of 255.255.255.128. So, the example internetwork requires 6 subnets and the available subnet is 8, this will allow for some additional growth in the network.

In the above table the network ID, first usable IP address, last usable IP Address and broadcast ID for all sub-networks are listed. You can see the borrowed bits are marked with red. Examine that borrowed bits in each network are the binary of that network number. For example, if we convert 5 into binary which is “101” you can see the borrowed bis in Net-5 which is the same to “101“