I am creating 128 sub nets for a organisation which has class C IP address, so i thought to chose 7 bits from the last octet of the IP address , and the rest that i am left with is one "0" which will be the host for each subnet , but the valid hosts are practically counting to zero.

If i chose a sub net mask : for a class C address , then total sub nets that i will have is 2^7 = 128 and hosts per sub net = 2 and valid hosts per sub net = 2-2 = 0.

So my question is what should we do if we want to have a 128 sub nets in our organisation? If i use the above method then i will have no valid IP in my sub net.


When using (31-bits) netmasks your "valid hosts per subnet" math sort of goes out the window here, because 31-bit masks are treated specially.

They're mostly used for point-to-point links, where there's no need for either a network address, or a broadcast, because each IP knows exactly where it's going to be sending all it's traffic.

It's actually even got it's own RFC (3021).

  • so what should i do if i want to create 128 subnets in a class C ip address? – Vishwa Ratna Jan 12 '17 at 13:30
  • Find more class C addresses, because you can't create 128 "normal" networks with a single class C. Also, this is starting to sound strangely like a test or homework question. What's your actual situation, and why do you need 128 exactly? – GregL Jan 12 '17 at 13:33

Most organisations use RFC1918 private addresses internally and only use registered addresses for external traffic.

  • so if some one ask me what is the host per subnet for my question? what should i say? 2 or 0? – Vishwa Ratna Jan 12 '17 at 13:20
  • tell them you can't divide it up that much, use private addresses internally – chicks Jan 12 '17 at 15:12

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