Classless routing uses a mask, or subnet mask, to define that point where the network portion stops and the host portion starts. The default subnet mask class for class A is 255.0.0.0, which says I have eight bits. The first octet is where the network portion stops. The mask can be defined two ways. One is by spelling it out just as we did here, 255.0.0.0; or there’s a short hand called a sider which just gives you the number of ones in the mask (/8). Either way works just fine.
Looking at the actual values of the mask, notice on the left that the lowest number we could have in any one of those octets would be 0. Keeping in mind it’s a series of 1’s starting from the left, the very next number we could have is 128.
If we wanted a 24-bit mask, it would look like this:
That’s a default for a Class C address. The mask tells us how far over to look to consider the host bits, or the network versus host.
How do we determine the size of a network? The size of the network is determined by the number of host bits. The more host bits you have, the larger your network will be. If you have one host bits it could either be a 0 or a 1. If we had two bits it could have 00, 01, 10, or 11, so we have four combinations with two bits. The size is determined by 2 to the number of bits. If we have 2 bits it’s 22 or 4 possible combinations. If we have 8 host bits it would be 28 which is equal to 256, and that would be the size of that network.
Guest Blogger: Jill Liles