Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cwjcc!ukma!rutgers!aramis.rutgers.edu!athos.rutgers.edu!hedrick From: hedrick@athos.rutgers.edu (Charles Hedrick) Newsgroups: comp.protocols.tcp-ip Subject: Re: ICMP Information Request Message format Message-ID: Date: 17 Mar 89 04:49:38 GMT References: <4483@ingr.com> Distribution: usa Organization: Rutgers Univ., New Brunswick, N.J. Lines: 29 I do not think you are confused. If Comer says ICMP Information Request can be used as a complete replacement for RARP, then he is confused. However it could be used in some cases. According to RFC792, ICMP Information Request can be used in the case where a host knows its own host number, but not its network number. The source address would be thus be something like 0.0.0.123. The host generating the reply fills in the address by supplying the local network number. In the early days, there were some schemes that mapped the host number into the low-order byte of the Ethernet address, derived it from the machine serial number, etc. In such a situation the local software might reasonably be able to deterine the local host number but not the network number. In practice I don't think anybody uses such schemes these days. So this technique for finding your own address isn't much use. I suppose one could extend ICMP information request by allowing a source address of 0.0.0.0 where a host didn't know anything about its address at all. If the system generating the reply can see the Ethernet address from which the request came, and if it has a table of Ethernet address vs IP address (i.e. the data needed to use RARP), it would then be in a position to fill in the full address. While one could imagine using such a technique, I've never heard of it being done. 4.3 BSD will work if you supply 0.0.0.123, but not 0.0.0.0. (If you did that, it would return an address like 128.6.0.0, i.e. the network number with the zero still in the low-order end.) In my view, that is technically what RFC 792 says to do. I suggest using BOOTP to find your IP address. Its advantage over RARP is that it will work even if you are separated from the bootp server by a router. It is also possible to us bootp to give configuration information other than just the IP address.