Monday, December 22, 2008

Prefix Notations - For CCNA

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:


255.255.255.0

or

/24

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:


11111111 11111111 11111111 00000000
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 at the beginning of 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...")


Be sure you're comfortable with prefix notation before taking your
CCNA exam. As with Cisco documentation, you'll most likely see masks
expressed in both dotted decimal and prefix notation, and you've got
to be ready to use the both as well!

Best Regards,
Deepak Arora

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