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I have two IP addresses. They share an IP address to the nth byte. Is there a word that effectively says that they have the same IP address up to the nth byte? Are there different words depending on how many successive bytes they have in common?

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First, an IP address is specified in bits, not bytes. Since humans tend to have a difficult time with binary, it is often converted into dotted decimal or bytes. This is an important distinction.

For example, two hosts share the same network of 10.10.10.0/23, they could have IP addresses of 10.10.10.3 and 10.10.11.3. They share the same 23 bits, but they only share the first two bytes, however the first two bytes will also include hosts that are not part of the network such as 10.10.255.3.

As to what the shared bits are called, this can be a number of things. Typically it will be network or net or subnetwork or subnet with or without ID or prefix appended (for example network ID or subnet prefix).

I am sure I have probably seen it called other things as well, but those combinations seem to provide everything I can think of at the moment.

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The word you are looking for is probably network prefix or subnet mask. It is used in conjuntion with an address mask.

For example:

192.168.0.0 / 24 or 192.168.0.0 / 255.255.255.0

"24" in means that the 24 most significant bits (3 bytes) of the ip address are the network prefix, the rest is available for addressing (without the .0 which is the subnet address and 192.168.0.255 which is a broadcast address for the network)

More on that topic on wikipedia: http://en.wikipedia.org/wiki/Subnetwork

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  • I wouldn't mind "network prefix" as an answer if it meant that two IP addresses with the same network prefix were the same address up to a specific length. – Adam Jensen Jun 25 '14 at 7:01
  • Are you clear about the difference between network prefix and subnet address ? – Kris Jun 25 '14 at 10:22
  • @AdamJensen: Regardless of your personal preference, network prefix or prefix length is the appropriate term. – bahamat Jul 1 '14 at 22:12
  • It is an appropriate term, and not necessarily what I was looking for. – Adam Jensen Jul 11 '14 at 6:09
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The term class N Network defines IP addresses that have the first bytes common.

Class A - only first byte common
Class B - first and second common
Class C - first to third common.
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    Class separates addresses depending on the leading bits, not bytes. – Adam Jensen Jun 26 '14 at 0:43
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    Please write 100 times: "Clasfull networking is dead an I shall only use Class-A, B C in an historic context." – Jens Link Jun 26 '14 at 9:44
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    @JensLink, only 100? I would go for at least 1000 since this is not a new change. It is always a pain to troubleshoot a device issue and find it is limited to "classful" addressing. In my experience, SCADA devices have been the worst offenders to this in recent memory. – YLearn Jun 26 '14 at 13:27
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    Was in a friendly mode when I wrote this. I would say 10.000. Just spend a couple of hours reading strace and tcpdump output. – Jens Link Jun 26 '14 at 14:02
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    The answer is not only obsolete but also wrong. Even when they were valid, before CIDR, classes corresponded to specific IP ranges, not only prefix lengths. See @YLearn's comment to another answer. – Dubu Jun 30 '14 at 13:27

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