jonathan@cs.keele.ac.uk (Jonathan Knight) (11/24/88)
Hi. At Keele we have a class C ethernet network (192.42.100)
which has a sequent, 2 microvax and several suns on it. I would
like to know what the correct broadcast address for our network
is. I have configured it as 192.42.100.255 but the sequent defaults
to 192.42.100.0. Which is the correct one to use?
--
_____ Jonathan Knight, || JANET: jonathan@uk.ac.keele.cs
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(_/ (_) / / Staffordshire. ST5 5BG. U.K. || ---------------- ukacrl.bitnetalmquist@JESSICA.STANFORD.EDU (Philip Almquist) (11/27/88)
Jonathan, > Hi. At Keele we have a class C ethernet network (192.42.100) > which has a sequent, 2 microvax and several suns on it. I would > like to know what the correct broadcast address for our network > is. I have configured it as 192.42.100.255 but the sequent defaults > to 192.42.100.0. Which is the correct one to use? Neither, actually. It used to be generally believed that your choice (192.42.100.255) was correct, but more recent wisdom, as codified in the forthcoming Host Requirements RFC, says you should use 255.255.255.255. Philip
mogul@DECWRL.DEC.COM (Jeffrey Mogul) (11/29/88)
Someone asked: > Hi. At Keele we have a class C ethernet network (192.42.100) > which has a sequent, 2 microvax and several suns on it. I would > like to know what the correct broadcast address for our network > is. I have configured it as 192.42.100.255 but the sequent defaults > to 192.42.100.0. Which is the correct one to use? Philip Almquist responded Neither, actually. It used to be generally believed that your choice (192.42.100.255) was correct, but more recent wisdom, as codified in the forthcoming Host Requirements RFC, says you should use 255.255.255.255. Right in theory, but in practice if you tell your 4.xBSD system to use 255.255.255.255 as a broadcast address, then I believe that it will not recognize incoming packets addressed to 192.42.100.255 as broadcasts. (This is from a quick reading of the kernel sources; I could be wrong, and I'd rather not do the experiment.) This might not be fatal (although in older 4.2BSD systems it might possibly lead to broadcast storms), especially if you configure ALL the systems on your LAN to use the same address, or if you don't care if some hosts cannot broadcast to some other hosts. I would specify the 192.42.100.255-style address with /etc/ifconfig, but write programs to ignore that and address their packets to 255.255.255.255. Then, if this fails, you should make your vendors fix their code (or upgrade to the latest release). Perhaps someone who believes it is safe to configure these systems with 255.255.255.255 as the kernel's idea of the local net broadcast address should correct me if I'm wrong. -Jeff