Path: utzoo!utgpu!water!watmath!clyde!bellcore!rutgers!gatech!bloom-beacon!tut.cis.ohio-state.edu!mailrus!uwmcsd1!ig!agate!ucbvax!DECWRL.DEC.COM!mogul From: mogul@DECWRL.DEC.COM (Jeffrey Mogul) Newsgroups: comp.protocols.tcp-ip Subject: re: default broadcast address Message-ID: <8807120110.AA15188@acetes.dec.com> Date: 12 Jul 88 01:10:00 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 44 First, try to follow the robustness principle: hosts should accept AS BROADCASTS all the possible (i.e., legal or formerly legal) broadcast addresses. For example, on a Class A network with an 8-bit subnet field, these addresses might be expected: 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)