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From: brian@anorak.utell.net (Brian Somers)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: Two devices with same IP
Date: 19 Nov 1996 13:40:14 -0000
Organization: Coverform Ltd.
Lines: 43
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Message-ID: <56sdbu$2ed@anorak.utell.net>
References: <56ltqk$nhv@csugrad.cs.vt.edu>
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In article <56ltqk$nhv@csugrad.cs.vt.edu>,
jagnew@csugrad.cs.vt.edu (H. Jared Agnew) writes:
: I have a friend that uses Redhat-4.0. He was giving me flac about the fact
: that FreeBSD will not allow the same IP on more than one device. I talked
: to a few friends and they told me that this was because haveing two devices
: with the same IP is against TCP/IP standards or something. If anyone knows
: where I could read more about this could you please post. Maybe a FAQ or
: RFC??? Thanks
There are two questions here.
1. Can two devices have the same IP.
Yes, as long as they can't "see" eachother. It's fine for my home
machine's LAN card to be called 10.0.1.1 as well as someone elses
home machine's LAN card - they can't see eachother. In a similar
way, it's fine to have a PPP interface with the same IP as a NIC
interface - but see question two.
If they get to see eachother, routing falls in a heap.
2. Can two interfaces have the same IP.
Yes, as long as at most one of them has a netmask that isn't 0xffffffff,
otherwise internal routing fails.
Interestingly enough, in 2.2-SNAP, ifconfig moans with a kernel printf
about another interface already having the IP that you've given it
as an arg. I think this is because the routing table is using the
IP as a unique index. From a "purist" point of view, I'd tend to think
that the same IP shouldn't really be allowed on two different interfaces,
after all, how can the kernel figure out where to send a packet destined
for that IP.
In practice, the last thing that you "ifconfig" with that IP is what
the kernel uses for "outgoing" packets, and all interfaces "ifconfig"d
with the IP can receive an "incomming" packet.
--
Brian <brian%anorak.coverform.lan@awfulhak.demon.co.uk>
<http://www.awfulhak.demon.co.uk/>
Don't _EVER_ lose your sense of humour....
.