Subnet Masks

As I mentioned in the previous chapter, subnets masks hold the key to understanding the breakdown between the network and host portions of an IP address. The reasons why the classes of IP addresses are split in the way they are is because of the subnet mask assigned to each class by default. The list below outlines the default subnet mask assigned to each class of address.

Class A:

Class B:

Class C:

Right off the bat, an association should become clear. Notice that for a Class A address, the default subnet mask occupies the entire first octet, with a value of 255. If you convert this subnet mask to binary, it becomes:

11111111 00000000 00000000 00000000

When the subnet mask in converted, the entire first octet is made up of binary 1s. The division between the network and host portion of a Class A address occurs where the 1s stop – in this case, right between the first and second octet. As such, we can say that when the default subnet mask is used, the address is separated as shown in the figure below.

Figure: Network and host portions of Class A address with the default subnet mask.

Notice the relationship between the subnet mask and the address. Think of a subnet mask as a cookie cutter that separates the network and host portions of an address. Without a subnet mask, you cannot make an educated statement about how the network and host portions of an address are defined. That may not be fully evident quite yet, given that we’ve only looked at classful addressing and default subnet masks. Later in the chapter, when we look at how custom subnet masks are defined, this distinction will become even more important.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.