Subject: Re: Default netmask
To: None <>
From: Wolfgang Rupprecht <>
List: tech-install
Date: 09/08/1998 11:58:37
> Broadcasts are done, in the end, to the ethernet broadcast address. In
> the end, that's all you need to know. If broadcasts didn't work before 
> you were configured, DHCP and friends couldn't work at all.

Getting the datagram to the router is only half the problem though.
The router also has to figure out what to send back to the host
requesting the information.

Admittedly, in general, there is only one IP network riding on each
ethernet.  Choosing the correct network to send back the netmask for
is easy enough.  On the other hand, the solution for the full-blown
case (where one has several IP networks riding on the same ethernet
wire) is impossible to determine without reverse-arp tables.  The
latter is what DHCP effectively does to figure out which of
potentially many networks the host is located on.

As a concrete example, the bridged-ethernet network this machine is on
(athome frmt1 ether) has 27 class-C networks on it, with over 5200
active hosts on it.  There would be no way for a router to be able to
figure out which netmask to send back to a machine unless it knew
either that machines IP address or had a valid ether-address to IP
mapping table.

If a router were to get a netmask request broadcast to IP address, I'd say that the most correct answer is /32
(eg. ;-)