Addressing in Networking

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It is one of the most important concepts as far as the networking is concerned. One can’t imagine maintaining a network without the proper knowledge of subnetting.

Subnetting is basically creating a multiple logical networks that exist within a single class of IP address. It is a powerful tool to conserve the fast depleting IPv4 addresses. Of course subnetting can be done in IPv6 address as well but we haven’t reached the threshold limit yet.

Subnetting is used to break the network into smaller and even more efficient “subnets” that prevents the wastage of the remaining IP addresses of a given network of a particular class. Subnet stands for “sub network”. Keep in mind that every subnet is independent and it behaves as if it is a separate network in itself. Routers treat them as if every subnet in an independent network and creates a separate entry in thr routing table to manage the network traffic accordingly.

I would like to elaborate this with the help of an example. Let us consider that a company named XYZ corp limited has its branches throughout the country.

This design requires 4 different network addresses. If IP of class C is taken into consideration viz

We can have 254 hosts on each side of the LAN interface of different routers in the whole network. You might be wondering where did 254 come from? Now I think it’s the correct time to introduce the formula for calculating number of hosts in a particular network:

(The reason for deduction of 2 will be discussed later on when we will discuss classful and classless network)

Applying the formula to the Class C IP address when subnetting is not done yields 254 (since number of host bits=8).

Also we require 3 IP addresses for connecting the respective WAN interfaces of routers.

Suppose Site A requires only 50 computers and its almost certain that they will never require more computers but the other two sites are growing rapidly and require to add almost 25 computers per month.

So after a while problem arises due to the lack of available IP addresses as there is one site which will never need more the 50 IP addresses and the remaining two sites can’t add more than 254 PC! The IP addresses that are going waste at Site A cant be used to add the computers for Site B and C.

The most teasing part is the network address used to connect the WAN interfaces of three routers present in the network. 251 IP addresss are literally going waste!

The ultimate aim of any company is to attain profit and minimize the expenditure. Every single IP address means spending a particular cost and thereby increase in the company’s expenditure. So we need to judiciously use the given network addresses.

In fact the network scenario discussed here can never happen in the reality. This is discussed just to elaborate the need of subnetting. Now lets again discuss the same network but this time let’s apply Subnetting and see the changes.

Instead of taking four different network addresses we took only one network address and applied subnetting. This way we are able to stop the wastage of IP addresses and we can use the remaining network addresses to those sites which are growing at a rapid rate.

Here we took the network address: 200.10.1.0/24 and applied subnetting and got 4 subnets that could be used on the four different sites viz

Note that the subnetting results in the changing of subnet mask. Here the default subnet mask of Class C changed from /24 to /26.

Subnetting is basically done by stealing the number of host bits and adding them in the network bits. Thereby the most obvious consequence is decrease in the number of available hosts in the network where subnetting is applied. Earlier we had 254 hosts available in Site A, B and C but now after applying subnetting they have reduced to 62 hosts per site but the biggest advantage is that we have successfuly conserved the other network addresses which we can now use elsewhere. Here we have used only one network address in the whole network as compared to 4 network addresses in the previous case.

Note: even this scenario is not used in practical life. This is just discussed to elaborate the advantages of a network where subnetting is applied over the network where subnetting is not applied. Practically VLSM (Variable Length Subnet Masking) is used which will be discussed later on.