Date: Wed, 7 Aug 2002 10:43:24 -0700 From: "Crist J. Clark" <crist.clark@attbi.com> To: Terry Lambert <tlambert2@mindspring.com> Cc: Brad Laue <brad@brad-x.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: ifconfig alias and the 0xffffffff netmask Message-ID: <20020807174324.GB71991@blossom.cjclark.org> In-Reply-To: <3D4F7F4F.97609D75@mindspring.com> References: <3D4F7539.2090201@brad-x.com> <3D4F7F4F.97609D75@mindspring.com>
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On Tue, Aug 06, 2002 at 12:48:31AM -0700, Terry Lambert wrote:
> Brad Laue wrote:
> > Minor question regarding this; to recap:
> >
> > Creating an IP alias on the same subnet as the original seems under
> > FreeBSD to require setting the netmask of the alias to 255.255.255.255,
> > or at least a subnet of the original.
> >
> > What impact, if any, will having a /32 netmask on an aliased IP have?
> >
> > It seems inconsistent with networking practice regarding interface
> > aliases, which typically view the aliased IP's simply as distinct hosts
> > on the same physical network, allowing them to have the same netmask.
> > This method is used with Cisco IOS and other Unix-like operating
> > systems. Is it incorrect?
>
> 255.255.255.255 means "This is an alias IP address".
>
> The actual netmask in effect is the same netmask as the real
> IP address.
>
> The thing that's broken is that you can't have a different
> netmask from that of the real IP address.
I've seen you say this before, Terry, and I'm not sure I understand
exactly what you are saying is broken. This,
ep0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 172.16.0.1 netmask 0xffff0000 broadcast 172.16.255.255
inet 172.16.255.254 netmask 0xffffffff broadcast 172.16.255.254
inet 172.16.2.128 netmask 0xffffff00 broadcast 172.16.2.255
ether 00:20:af:17:0f:11
media: Ethernet 10baseT/UTP
Works fine for me. What do you mean by, "you can't have a different
netmask from that of the real IP address?" In the case of a
255.255.255.255 netmask, you have a different netmask from the "real"
IP address. That's the whole point of using the all-ones mask after
all.
--
Crist J. Clark | cjclark@alum.mit.edu
| cjclark@jhu.edu
http://people.freebsd.org/~cjc/ | cjc@freebsd.org
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