Path: utzoo!attcan!uunet!ogicse!decwrl!ucbvax!SERVER.AF.MIL!jonson From: jonson@SERVER.AF.MIL ("Lt. Matt Jonson") Newsgroups: comp.protocols.tcp-ip Subject: Re: IP numbers that end in 0 ... Message-ID: Date: 28 Aug 90 00:05:44 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 59 In message <63256@bu.edu.bu.edu> usc!zaphod.mps.ohio-state.edu!rpi!bu.edu!bu-it!kwe@ucsd.edu (Kent England) writes: > I didn't see anybody cite chapter and verse of the Host Requirements RFC >and considering the hours spent on that fine document, I think it would >be a shame >not to cite the official bible of the Internet (ok, one of the bibles...): > >----------------------------- > From RFC 1122 Sec 3.2.1.3 Addressing: RFC-791 Section 3.2; > > "IP addresses are not permitted to have the value 0 or -1 for > any of the , , or number> fields (except in the special cases listed above). > This implies that each of these fields will be at least two > bits long." >------------------------------ > [...] >I therefore conclude that a host part of zero is never a legal source address. >So I think the petitioner should change the addresses on his hosts and >not bother his >router vendor. Isn't the Host Requirements RFC wonderful? This only >took five minutes >to research and that included checking the commentary in RFC 1127. :-) And this is all correct, you cannot have a of 0. However, you can have a final OCTET of 0 legally if you are subnetting (for instance) a class B on LESS THAN 8 bits. For instance, 5 bit and 11 bit from class B: BBBBBBBB.BBBBBBBB.SSSSSHHH.HHHHHHHH 10000011.00111111.00001001.00000000 This should be a legal address, because it is on subnet 1, and is host #256. The dotted decimal notation is 131.63.9.0 !!! which is the type of situation that I believe the person who posted the question was talking about. This is a legal number, but there may be some old broken routers out there that still think it is bad because they don't know subnetting (?) Similary, one could generate an address that (dotted-decimally) ended in 255, that should also be technically legal, but may just as well be filtered by a gateway that didn't know any better. Conclusion: May be wise to avoid creating host addresses that have an all 1 or all 0 fourth octet if you don't have the time to hunt down routers that filter on what they assume is an errant broadcast. Contrarily, it would be nice if someone actively pursued broken implementations... (Lt) Matt Jonson AF DDN Program Office jonson@server.af.mil And yes, those were my words, not the government's. (They don't pay me enough to speak for them too)