Possible Duplicate:
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?
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.
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.
