Re: default broadcast address


Phil R. Karn (thumper!karn@faline.bellcore.com)
12 Jul 88 21:03:48 GMT


The "correct" IP address to use in broadcast packets seems to have
become a perennial issue. So once again I'll point out that, in my
opinion, hosts should treat any packet as a broadcast if it comes in
with the LINK level broadcast address (e.g., all 1's on Ethernet). The
IP destination address field in such packets should be completely
IGNORED.

An "out of band" indication of whether a given packet is a broadcast or
not should be passed up along with the packet to the IP layer, and from
there to the various transport protocols. (Certain protocols, e.g. TCP,
have good reason to refuse to respond to broadcast packets).

This is the only way I know to keep gateways from forwarding broadcasts
from hosts with the "incorrect" IP broadcast address. When you think
about it, much of an IP header (including the destination address field)
is meaningless in the context of a broadcast packet anyway since
broadcasts are not supposed to pass through gateways.

Phil



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