re: default broadcast address


Jeffrey Mogul (mogul@decwrl.dec.com)
11 Jul 1988 1810-PDT (Monday)


SCOPE ILLEGAL LEGAL
---------------- ---------------- ------------------
wildcard 0.0.0.0 255.255.255.255
net-broadcast net.0.0.0 net.255.255.255
subnet-broadcast net.subnet.0.0 net.subnet.255.255

Note that this is especially important because there are some things
that a host must never do in response to a broadcast packet (such as
send an ICMP Destination Unreachable), and so these must all be recognized
as broadcasts (this is even more important than accepting them).

Second, hosts should never send all-zeros addresses (unless the
LAN administration has decided to temporarily disobey the standard
until they can find a vendor that sells proper software).

Third, in a world where rules 1 and 2 are obeyed, most hosts should
be configured to send broadcasts to 255.255.255.255, to avoid having
to configure one more thing on a per-host basis. The exception here
is that on multihomed hosts (and on gateways) there may be applications
which want to broadcast to one, but not all, of the connected subnets.
On these hosts, broadcasts should be addressed to the subnet-broadcast
addresses.

Fourth, I don't know of any implementations of the reverse-path-forwarding
mechanism I proposed in RFC922, so there is probably no reason to use a
net-broadcast address on a subnetted network. Again, this assumes that
everyone is playing by the rules.

Finally, it appears true that on most LANs, rules 1 and 2 are violated
by at least one host. In such cases, the best solution is to pick a
broadcast address that all the hosts can live with, and resign yourself
to the occasional broadcast storm or dropped broadcast.

An important point is: there is a big difference between what a host
should recognize/accept (everything), what a host should send (legal
things), and what should be used (the most legal thing for which every
host won't screw up and probably will accept).

-Jeff Mogul (mogul@decwrl.dec.com)



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