Re: How do you break up a B class number?


J Q Johnson (jqj@gvax.cs.cornell.edu)
Fri, 31 Jul 87 14:30:06 EDT


Dan Lynch <LYNCH@A.ISI.EDU> suggests (in jest, I think) a solution to
the heterogenous subnet problem:
> . . . have more than one entwork number on the same "cable"! So break
>the 16 bits up into 256 nets of 256 hosts and assign 4 of the to the
>main cable. It's legal , but wil it work!?

No, it probably won't work. One big problem is that you are likely to
have broadcasts with all sorts of broadcast addresses. Suppose that we
have 128.84.253.0 and 128.84.33.0 (netmask 0xffffff00) on the same cable.
Then the host with interface address 128.84.253.3 will occasionally
receive Ethernet broadcasts that contain IP broadcasts with destination
128.84.33.255. If this is a typical 4.3BSD implementation, it will say
"that's not a broadcast address, so I gotta forward or send an ICMP
unreachable or something". Result: every host on 128.84.253.0 replies
at the same time, and you get a big Ethernet collision. We tried something
like this, and sure enough our SUNs were reporting 70% collision rates!

Another version of the Ethernet meltdown Charles Hedrick so aptly described
in these pages a few weeks ago.



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