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I was watching a tutorial video on Lynda.com. The presenter said:

  • Class A: First octet between 1 and 126.
  • Class B: First octet between 128 and 191.
  • Class C: First octet between 192 and 223.

I did some Google searching, and according to Wikipedia,

  • Class A starts from 0.0.0.0 and ends with 127.255.255.255.
  • Class B starts from 128.0.0.0 and ends with 191.255.255.255.
  • Class C starts from 192.0.0.0 and ends with 223.255.255.255.
  • Class D starts from 224.0.0.0 and ends with 239.255.255.255.
  • Class E starts from 240.0.0.0 and ends with 255.255.255.255.

I found another similar course on Udemy. The presenter said:

  • Class Address Range
  • A 1.0.0.1 to 126.255.255.254
  • B 128.1.0.1 to 191.255.255.254
  • C 192.0.1.1 to 223.255.254.254
  • D 224.0.0.0 to 239.255.255.255
  • E 240.0.0.0 to 254.255.255.254

I cannot ask the presenters directly, so my questions are:

  • Should 127 be included in Class A or not?
  • Why does the address range vary (sometimes from 128.1.0.1, sometimes from 128.0.0.0, as well as end with 254 or 255)? Which one is correct?

I am surprised that this classful addressing must have been around for years, why there is not a unified definition? Sorry about the wordy questions and thanks in advance.

1

Yes, 127.x.x.x should be included as a Class A address.

128.1.0.0 - 128.1.0.255 is the correct entire network block for a /24 network.
128.1.0.1 - 128.1.0.254 are the correct usable hosts for a /24 network.
128.1.0.0 is your network.
128.1.0.255 is your broadcast address.

You will sometime see the first IP (128.1.0.0) and last IP (128.1.0.255) left off when documenting the usable host addresses.

  • Thank you for providing such a succinct yet detailed answer. Now I understand why variations exist in different versions. – Pesticide Jan 26 '17 at 4:01
5

Classful networking is dead, and it has been for over 20 years, killed in 1993 by RFCs 1518 and 1519, which defined CIDR (Classless Inter-Domain Routing). Please, let it rest in peace. Modern networking does not use classes.

If you insist on using outdated, obsolete classes, IPv4 addresses are 32-bit integers. If you remember RFC 870, ASSIGNED NUMBERS:

  • Class A networks have the first bit as 0.
  • Class B networks have the first two bits as 10.
  • Class C networks have the first three bits as 110.
  • Class D networks have the first four bits as 1110.
  • Class E networks have the first four bits as 1111.

You should be able to figure out the old network class for any address with the above information, not that it means anything, anymore.

  • Thank you so much for editing my post and answering my questions. I will let it go and just learn about CIDR. – Pesticide Jan 25 '17 at 2:18
  • If this answers your question, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. – Ron Maupin Jan 25 '17 at 2:26
  • I didn't realize I should accept an answer. Sorry about not closing the question sooner, and I want to choose the other one. It just addresses my questions more specifically. Thank you for taking the time to reply and letting me know what I was supposed to do after a question was responded. – Pesticide Jan 26 '17 at 3:56
  • That is fine, as long as your question is answered. We aim to please, but a question will keep popping up to the top as long as it doesn't have an accepted answer. If we haven't answered, you can comment and ask for clarification. – Ron Maupin Jan 26 '17 at 3:59

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