Now that we have spent a lot of time working out way through the math behind a subnet mask and how to calculate ip addresses, why do we need a subnet mask? What is it's purpose? A subnet mask can be used to break a network into smaller sections, or subnets. Why would you want to do this? Since there are limited number of ip addresses, you're Internet provider will likely provide you a single address range according to your estimated needs. So if your network needs to be broken up into smaller subnets to either reduce traffic, or for security reasons, then you must break it into subnets.
What are 4 of the major reasons for subnetting or segmenting your network?
To divide a large network into smaller segments to reduce traffic and speed up the sections of your network.
To connect networks across geographical areas.
To connect different topologies such as Ethernet, Token Ring, and FDDI together via routers.
To avoid physical limitations such as maximum cable lengths or exceeding the maximum number of computers on a segment.
In the next two sections you will see examples of the different sized subnets you may use.