[302] in linux-net channel archive
Re: Re^2: Linux-Activists - NET Channel digest. 95-4-9-6:26
daemon@ATHENA.MIT.EDU (Rob Janssen reading Linux mailingl)
Wed May 10 09:31:47 1995
To: eowmob@exp-math.uni-essen.de (Michael Weller)
Date: Wed, 10 May 1995 10:08:33 +0200 (MET DST)
Cc: middelin@polyware.iaf.nl, linux-net@vger.rutgers.edu,
linux-activists@joker.cs.hut.fi
In-Reply-To: <Pine.3.89.9505091255.A54903-0100000@werner.exp-math.uni-essen.de> from "Michael Weller" at May 9, 95 12:51:15 pm
Reply-To: linux-vger@wab-tis.rabobank.nl
From: linux@sys3.pe1chl.ampr.org (Rob Janssen reading Linux mailinglist)
According to Michael Weller:
> > An single point can be used for a slip line, which has only
> > one IP address... An netmask of 255.255.255.255 is valid for
> > this one-netter.
>
> Uhm. nope. A slip interface has always exactly two reachable ip
> addresses. to stay with above example: 193.170.67.253 and 193.170.67.190.
>
> Above ifconfig makes both addresses valid. A netmask of 255.255.255.255
> would make one of both unreachable and make the slip line unusable. Using
> another netmask will usually look like if there are other people
> reachable directly via this interface to (except when both ip addresses
> only differ in the last bit) which is horribly wrong either.
>
> A netmask is required for networks where you can directly reach more than
> one other partner (not counting yourself). There you must tell your
> machine which range of ip-addresses can be directly reached with this
> interface.
>
> There is nothing like that for slip. There are two fundamental types of
> interfaces, that is pointtopoint (which does not have a netmask) (say
> SLIP, PPP, PLIP) and those that are not pointtopoint (which need a netmask)
> (ethernet, tokenring, FDDI, ...)
>
> Usually the value of netmask is simply ignored for pointtopoint interfaces.
>
> Thus one could claim that above netmask ifconfig should simply fail
> telling that sl0 is in pointtopoint mode w/o a netmask.
I think the correct way of setting it up is to use 2 IP addresses from
a 4-address subnet, i.e. the addresses ending in binary 01 and 10.
The subnet mask for that network is 255.255.255.252
Valid addresses would be:
x.x.x.1 and x.x.x.2
x.x.x.5 and x.x.x.6
etc.
Setting it up like this allows autorouting protocols that trust the
netmask to work correctly without further configuration.
Rob
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