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


J Q Johnson (jqj@gvax.cs.cornell.edu)
23 Jul 87 11:05:44 GMT


In article <In article <11636@hi.UUCP> kurt@hc.dspo.gov (Kurt Zeilenga) writes:
>Does anyone know of any good references for netmasking (subnets)
>schemes to split a B class number into various sized networks?

Although variable-sized subnets may work for some network topologies,
they are almost guaranteed to get you into trouble, and I strongly
recommend that you avoid them. Consider a subnet (perhaps the backbone)
with two or more gateways. Host A on this subnet wants to send a packet
to host B on a different subnet. In order to look up the route to B,
A needs to decompose B's address into net-subnet-host, so he needs to
know B's subnet mask. All current software that I know of will use
A's mask, and ASSUME that it is the same size as B's.

Granted you can fool the routing tables in some topologies, e.g. a
network with subnet mask of 255.255.255.0 containing several subsubnets
(who think the subnet mask is 255.255.255.188 or something) all
connected to only a single (hacked) gateway, where that gateway
advertises a subnet that is the union of the subsubnets. It will break
as soon as you make the topology more complex!

Given that we can't do what Zeilenga asks, is it perhaps time to rethink
the whole subnet scheme? 16 bits of hostnumber is not much at all for
a typical large organization (say a university), especially if we have
to waste most of it because of subnet constraints.



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