freiss@nixdorf.de (the hacker) (11/08/90)
This might be a silly question, but it's been worrying me for some time. What happens if different IP subnets live on the same ethernet cable? I'm worried about excessive ICMP traffic (ICMP net_redirect packets); I see some SGI workstations here react to some packets from a different subnet with this ICMP message. Could somebody tell me if this is a problem or what other problems I haven't even thought of might arise? Pointers to books, RFCs etc. on this subject are also welcome. Thanks, Martin -- Martin Freiss | internet: freiss.pad@nixdorf.com SNI AG | dnet: freiss.pad@nixdorf.de Dept. STO-NC 333 | Voice: +49 5251 14 6601 D-4790 Paderborn | fun: marvin%dg5kx@infodn.rmi.de
kwe@buit13.bu.edu (Kent England) (11/10/90)
>What happens if different IP subnets live on the same ethernet cable?
This question comes up often and bears mention occasionally.
Don't forget that what happens with subnets depends on who is looking
and how they are configured.
If you subnet your (eg) Class B with eight bit subnet masks
and you don't want (can't have) variable length subnets, then subnet
layering looks attractive for configuring your routers.
But your hosts can see things differently than their routers.
If you layer subnets, you may set the subnet mask on these layered
subnet hosts to be the network mask. They will then ARP for all
members of their (subnetted) net and communicate directly to all their
directly attached neighbors with no extra hops, failure points or
extra traffic and the routers will answer for off-net members of the
subnetted net, if your routers are set to proxy ARP.
It works for 4.2BSD and it works well for layered subnets.martino@logitek.co.uk (Martin O'Nions) (11/12/90)
freiss@nixdorf.de (the hacker) writes: >This might be a silly question, but it's been worrying me for some time. >What happens if different IP subnets live on the same ethernet cable? >I'm worried about excessive ICMP traffic (ICMP net_redirect packets); >I see some SGI workstations here react to some packets from a >different subnet with this ICMP message. >Could somebody tell me if this is a problem or what other >problems I haven't even thought of might arise? >Pointers to books, RFCs etc. on this subject are also welcome. Observe caution! RFC950 adds a couple of new ICMP types, including one to allow determination of subnet mask for this segment. This specifically states that a host may request the active subnet mask EVEN IF IT DOES NOT KNOW ITS OWN IP ADDRESS (IP 0.0.0.0 (!?**@!)). Therefore, even though it it is (hopefully) unlikely to occur, this section of the specification could cause prob's if someone chose to implement the RFC in this detail. (P.S. If anyone has any info. which updates/obsoletes/contradicts this, don't flame me - mail me!) -- +------------------------------------------------+-----------------------+ | I've got a little black book with my poems in, | Martin O'Nions | | I've got a bag with a toothbrush and a comb in,| Logitek Group Support | | When I'm a good dog they sometimes throw me a | martino@logitek.co.uk | | bone in (Roger Waters - The Wall) | 0257 426 644 | +------------------------------------------------+-----------------------+
mperlman@Encore.COM (Mark Perlman) (11/13/90)
In article <martino.658404029@krypton> martino@logitek.co.uk (Martin O'Nions) writes: >freiss@nixdorf.de (the hacker) writes: > > >>This might be a silly question, but it's been worrying me for some time. >>What happens if different IP subnets live on the same ethernet cable? > >>I'm worried about excessive ICMP traffic (ICMP net_redirect packets); >>I see some SGI workstations here react to some packets from a >>different subnet with this ICMP message. > >>Could somebody tell me if this is a problem or what other >>problems I haven't even thought of might arise? >>Pointers to books, RFCs etc. on this subject are also welcome. I may have missed some of the previous posting in response, however, I'll put in my 2 cents for free. You can "turn off" subnetting by specifying a metric of 0 (zero) when you use the route command. E.G.: route add <some_subnet_address> <some_host_address_in_your_subnet> 0 You also need to consider broadcasting addresses on this multi-subnetted physical network. A broadcast address of xx.xx.xx.255 says "all hosts on subnet xx.xx.xx.0", whereas, a broadcast address of 255.255.255.255 says "all hosts an my local network". It requires some forethought on your part to assure a working broadcasts. I would reccommend getting your hands on Charles Hedrick's paper, "Introduction to Administration of an Internet-based Local Network", 3 October 1988. Hope this was useful. +----------------------------------------------------------------------+ | Mark R. Perlman | | Independent Consultant 301-206-2016 | | 14014 Oakpointe Dr. mperlman@encore.com | | Laurel, MD 20707 uunet!gould!mperlman | +----------------------------------------------------------------------+