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Is X.Y.Z.0 a valid IP address?

Maybe a noob question but when referring to IP subnets, what is the purpose of a network IP.

i.e. with a network like 192.168.1.0/24, you can't normally use the .0 for a host address. Likewise the .255 is assigned to the broadcast. This I understand but the .0 I do not. What is the purpose of it and why are point to point links with /31 mask bits able to do away with it?

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marked as duplicate by voretaq7 Nov 19 '12 at 23:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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I am fairly certain that use of /31s was a change to the original RFC or an extension. The /31s are the exception to the rule. –  SpacemanSpiff Nov 19 '12 at 22:28
    
tools.ietf.org/html/rfc3021 –  SpacemanSpiff Nov 19 '12 at 22:29
    
    
You should probably say point-to-point not peer to peer. On a point-to-point link there really is no need for broadcast. –  Zoredache Nov 19 '12 at 22:31

2 Answers 2

Possible duplicate or closely related to Is X.Y.Z.0 a valid IP address?.

Short answer is that it can be a valid host address, depending on the subnet mask. It was historically used as just a network identifier or broadcast address, but there was no technical reason to let a .0 address (or subnet) go to waste, and it has become more widely used. You can read a bit more about it here from Cisco, but the basic gist is that the .0 address and subnet and the "all ones" subnet were supposed to be reserved as special, per RFC950, even though they were always usable.

That practice (of excluding the 0 and "all ones" subnets from assignment) is considered an obsolete practice now, per RFC1878.

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Great, so suppose I have a network with 25 mask bits. I can use the bottom address for the router? –  Matt Nov 19 '12 at 22:53
    
And you say, it depends on the subnet mask. What restrictions are there? –  Matt Nov 19 '12 at 22:55

In the old days, host 0 was actually used as the broadcast address for a network. However, that evolved over the time into having the max host number being the broadcast address. There was a period of time where BOTH were acceptable and most likely why these two host numbers are reserved in the standard. I even remember having to configure ethernet cards and specifying the broadcast address for this reason to make various heterogenous network equipment work properly.

Over time everybody decided that the max host number was best, and identification of host number 0 was redefined.

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