In class ful addressing: The network bits must match: For class A first octect For class B first two octects For Class C first three Octects

What about Classless addressing? Network ids derived from subnetmask. Is it mandatory to match first three octects in, ipaddress?

In classless how systems feels they are on same network? On what criteria they derive network address?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 14, 2017 at 14:23

1 Answer 1


First: Classful addressing is obsolete and is no longer used. It was obsolete (I'm guessing) since before you were born. A pox on schools that keep teaching it.

Second: Dividing an IP address into octets is just for the convenience of human readers. To the computer, it is just a 32-bit address. There is no meaning to the octets other than it makes it easier for humans to read big binary numbers.

Third: The subnet mask defines which bits are the network ID and which are the host ID. The 1's bits indicate the network portion of the address.

  • "There is no meaning to the octets other than it makes it easier for humans to read big binary numbers." Based on my personal experience, don't ever say that on Server Fault or other SE sites; you will be shouted down. :) Some people just don't get it.
    – Ron Maupin
    Jun 9, 2016 at 17:10
  • A pox on them too ;-p
    – Ron Trunk
    Jun 9, 2016 at 17:12

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