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Last Post by CimmerianX
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Class A networks, start from 1.0.0.0 to 126.0.0.0.

Class B network addresses start from 128.0.0.0 to 191.255.0.0.

Class C network addresses start from 192.0.0.0 to 223.255.255.0.

Hope that helps

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So, You check the range in which the network address you are checking fits, and then you will know what class it is.

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If I may add some additional information to help clarify the thread...

What actually determines an IP's class in a classful network is actually the IP addresses's leading bits.

All Class A addresses will have first leading bit as zero (0). So, actually Class A's range is:
0.0.0.0 - 127.255.255.255.

What I mean by the leading bit is, if you convert the decimal value to binary for the first octect, you have 8 places. A Class A address must have a 0 in the first bit. In binary the range is:

00000000 - 01111111 which in Base 10 --> 0 - 127. We know that 127.0.0.1 is the loopback address, but it is still classified in the Class A range.

A Class B address, the first and second leading bit must be a '10' in binary. So for example, the range is: 10000000 - 10111111 which equals in Base 10 --> 128 - 191.

A Class C address, the first, second, and third bit must be '110' in binary. So for example, the range is: 11000000 - 11011111 which equals in Base 10 --> 192 - 223.

So, in summary, here are the Classes of A-C:

Class A     0.0.0.0     127.255.255.255
Class B     128.0.0.0   191.255.255.255
Class C     192.0.0.0   223.255.255.255

If you are interested in Class D and E, just continue with the pattern of leading bits.

Class D --> '1110'
Class E --> '1111'

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