two public ip addresses on one interface

[Wilkinson, Alex]

    0n Fri, May 25, 2007 at 01:17:39PM -0700, Chuck Swiger wrote: 

    > If your aliases are part of the same subnet as the "primary" or first configured IP, then you want to 
    > use the all-1's netmask.  In your case, however, the second IP is part of a completely different subnet, 
    > and you can (and should) use a /24 netmask....

I have always wondered why we need to use the "all-1's" netmask. Why is this ?

 -aW

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