When you’re preparing to pass the CCNA exam and earn this coveted Cisco certification, you’ve got to be totally prepared for the many kinds of binary and subnetting questions Cisco may throw at you. You also have to be familiar with the different manners in which a subnet mask can be expressed, and that’s where your knowledge of prefix notation comes in. Prefix notation is an alternate way to express the value of a subnet mask, as opposed to the more familiar dotted decimal format. Not only will you see prefix notation in Cisco documentation, but you’ll probably see it on your CCNA exam. Consider the following two values:
Believe it or not, those two values are exactly the same. The first mask is written out in the more familiar dotted decimal format, and you know by looking at those first three octets that every bit is set to “1”, since the maximum value of such an octet is 255.
The second value represents the exact same mask, only this value is expressed in prefix notation. This particular value would be pronounced “slash twenty-four”, and the 24 represents the number of consecutive ones that are set in the subnet mask.
Those of us who hate to type numbers are particularly appreciative of this, since it means you’ll have to type a lot less numbers to represent a subnet mask. In addition, it’s a lot easier to discuss masks in prefix notation than dotted decimal. (“I thought about using a two-fifty-five two-fifty-five two-fifty-five zero mask ,but then decided to use a two-fifty-five two-fifty-five two-fifty-five one-twenty-eight mask…”)