variable sized subnets


J Q Johnson (jqj@gvax.cs.cornell.edu)
Fri, 24 Jul 87 09:37:18 EDT


In article <In article <11636@hi.UUCP> on comp.protocols.tcp-ip, 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 one avoid them.

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.

Why don't variable sized subnets work? Routing and broadcast. Broadcast
isn't too bad as long as all hosts have the "correct" subnet mask set for
their size of subnet and gateways don't leak broadcasts off the local
subnet, but routing is a problem. 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 (for 4.3BSD, the mask for the first interface on that network!), 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!

Am I missing something? Has anyone successfully made variable-sized subnets
work in a real (multivendor, multisubnet) environment? Does anyone have
suggestions for a clean way to make them work?



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