IP Aliasing

Jon Radel jon at radel.com
Fri Apr 11 01:48:26 UTC 2008

David Allen wrote:
> This may be a dumb question, but I'm puzzled by the following (taken
> from the Virtual Hosts section in the Handbook):
> 	For example, consider the case where the fxp0 interface is connected
> 	to two networks, the network with a netmask of
> and the network with a netmask of
> IP aliasing I get, but two different networks on the same interface?
> What would this be plugged into to make that work?

Ethernet most likely these days.

In a perfect world, where ipv4 addresses flowed like water, everyone
managed to forecast everything perfectly, and nobody ever had to
renumber a network, I doubt there'd be much call for it.  And I'd never
want to try make a case for it being terribly elegant.

I'm personally acquainted with a couple of cases where it comes up:

1) Multi-homed networks with ipv4 addresses assignments too small to do
something "real" like using BGP to advertise >= /24 to multiple ISPs.
So to talk via one ISP you use one address and via the other ISP you use
the other.

2) You need to migrate to new addresses but can't afford to shut
everything down long enough to change everything all at once.

There are others.

--Jon Radel
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