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Subnetting Made Easy – Part 2 Classful Addressing

Author: Guest Authors 12 March 2012 2,970 views No Comments
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In IPv4 we have 32 bits divided into 4 octets. Now, of the 32 bits we need to determine where the network portion is and the host portion is, so there’s two pieces to the 32 bit address two ways of defining that. One way is what’s known as classful, and our classes are A, B, and C. The second way is class-less where we use a subnet mask.

With classful addressing, we use the first octet to tell us if we have a 0 in th first bit, which is the very first bit of the first octet. That means that this would be a class A address. So, the smallest number we could have in a class A address would be all zeroes:

0 0 0 0 0 0 0 0

The largest number we could have in this octet would be a zero and a one, then all ones, so an A address in this first octet means it’s between 0 and 127.

0 1 1 1 1 1 1 1
128 64 32 16 8 4 2 1

64+32+16+8+4+2+1 = 127

Our class B address must have a 1 in this first position and a 0 in the second position, so the smallest number we can have in a class B address is 128:

1 0 0 0 0 0 0 0

The largest number, with a 1,0 then all ones, is 191. So a class B has a 1,0 pattern in the very first octet and so that would be between 128 and 191.

1 0 1 1 1 1 1 1
128 64 32 16 8 4 2 1

128+32+16+8+4+2+1=191
Class C address says we have to have a 1,1,0 pattern in the first three bits of our very first octet. So the smallest number would be 1,1,0 and all zeroes; that’s equal to 192:
11011111
And then a 1,1,0 then all ones following that, equals 223. So this is our class C range: 192 – 223:

1 1 0 1 1 1 1 1
128 64 32 16 8 4 2 1

128+64+16+8+4+2+1=223

Classful addressing states that if the first octet is between 0 and 127, it’s a class A and the remaining 3 octets are all host bits. A class B address states that the first octet is between 128 and 191, the first two octets are the network portion, and the last two octets are the host portion. A class C address has between 192 and 223 in the first octet, the first three octets are the network portion, and the last octet is the host portion.

So classfull addressing looks at the first octet to determine the class and the size of the network portion of the address and the host portion of the address.

Guest Blogger: Jill Liles

Subnetting Series

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