Step-by-Step of Subnetting an IP Address

Subnetting an Internet Protocol (IP) address is the process by which a network engineer, in charge of a certain TCP/IP network number, alters the existing network rules. The IP is the numerical address assigned to your computer while TCP/IP gives a description of a set of guidelines that make computers able to communicate. Subnetting of IP addresses provides you with the solution of dealing with large numbers. For instance, class A has more than 16,000,000 IP addresses while class B has 65,000. Therefore, if subnetting is not done, the number of IP addresses will be very large and that means they will not be fully utilized.

Instructions

1. Calculate the number of subnets required. These numbers of the subnets will rely on the type of network you have chosen to use. For example, if you are using a network that has three routers connected through serial links, then each shared link will need a single subnet. Serial links are ports that provide your computer with interface connection.

2. Find out the number of bits that you can borrow. In this case bits are the smallest units of data in your computer. But take note that the number of bits changes on the basis of the type of network address selected. For instance, if you started with class A, you need to have 24 Host ID bits that are identifiers unique to your computer, and you can borrow no more than 22 of them.

3. Calculate the number of host ID bits you can borrow. This is important because you will get to know the required number of subnets in the network you are using. According to Jennifer DeHaven Carroll and Jeff Doyle in their book "Routing TCP/IP," the formula for calculating this is 2n - 2 = #, where n is the number of borrowed bits.

4. Turn on borrowed bits and calculate the decimal value. Using the number of bits you can work with and also borrow, begin with high order bits by starting on the left of a binary number.

5. Calculate the new subnet mask by adding the decimal value to the default subnet mask for the type of address you are subnetting. In this case a subnet mask is the 32-bit number that's masks and divides the IP address into host address and network address.

6. Find out the lowest of the high-order digits starting from the left side. Select the host variable before creating the subnet values for your IP address.

References

• "The TCP/IP Guide: A Comprehensive, Illustrated Internet Protocols Reference"; Charles M. Kozierok; 2002
• "Routing TCP/IP"; Jeff Doyle, Jennifer DeHaven Carroll; 2001
• "Mastering Windows Vista Business: Ultimate, Business, and Enterprise"; Mark Minasi, John Paul Mueller; 2005
• CISCO: Subnetting Examples