Date: Tue, 27 Feb 2001 07:16:14 +0100 From: "Rogier R. Mulhuijzen" <drwilco@drwilco.nl> To: "Matthew Emmerton" <matt@gsicomp.on.ca>, "Jonathan Graehl" <jonathan@graehl.org>, <freebsd-net@FreeBSD.ORG> Subject: Re: Quick question about IP aliasing Message-ID: <4.3.2.7.0.20010227070424.00d25c10@mail.bsdchicks.com> In-Reply-To: <001501c0a070$3a14d900$1200a8c0@gsicomp.on.ca> References: <NCBBLOALCKKINBNNEDDLAEJIDLAA.jonathan@graehl.org>
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At 22:49 26-2-01 -0500, Matthew Emmerton wrote:
> > > do 'netmask 255.255.255.255' instead or 'netmask 0xffffffff' since this
>is
> > > an alias... for some reason otherwise services may not bind to the ip
> > > correctly
> >
> > Why would this be? The two are numerically equivalent.
He's saying 'instead or' not 'instead of', I nearly fell for this too.
>Yes, but you're missing the point.
>
>The point is that you need to use a netmask of 255.255.255.255 for aliased
>IPs on FreeBSD, regardless of the alias of the primary (non-alias) IP.
Everybody is saying use 255.255.255.255 for an alias. Noone is giving
reasons why.
Aliassing is the only way to bind more than one IP to an interface (well
only simple way..) and aliasses are no different from the primary IP.
If you couldn't use the right subnetmask for an alias outside the main IP's
subnet, the FreeBSD machine wouldn't be able to fully participate in that
subnet's traffic. (It couldn't receive subnet broadcasts for one)
The only reason to use 255.255.255.255 is when the IPs are in the same
subnet, or you'll get routing and broadcast problems.
If you use 10.1.1.1:255.255.255.0 and the 2nd IP is 10.1.1.2 you need to
use 255.255.255.255 for the 2nd IP
If you use 10.1.1.1:255.255.255.0 and the 2nd IP is 10.2.2.2 you can use
255.255.255.0 for the 2nd IP (if that is the subnetmask for that network)
If you use 10.1.1.1:255.0.0.0 and the 2nd IP is 10.2.2.2 you need to use
255.255.255.255 again (same subnet)
If you use 10.1.1.1:255.255.255.0 and 10.2.2.2:255.255.255.0, and the 3rd
IP is 10.2.2.4 you use 255.255.255.255 only for the 3rd IP.
DocWilco
P.S.: If this is not the case with FreeBSD, we've got major problems.
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