[PLUG] RE: mixing netmasks

[Jeff Moore]

Somebody I know is proposing to mix netmasks on the same physical
network.  That is to say all machines that are supposed to talk to each
other (servers and clients of those servers) would be in 10.1.1.0/24.
Engineering machines (prototypes, machines under test) would be in
10.1.0.0/16 in the 10.1.2 range and engineering workstations, which
needed to talk to the servers and the test macnines would be in the
10.1.1.0 range with a netmask of 255.255.0.0

I told him that that was a bad idea, and I think it would cause
problems.  Am I talking through my hat again? (most of the machines on
the 10.1.1.0/24 net are M$ boxen and most of the engineering
workstations are Linux.

It certainly is a bad idea. First thing to consider is that each subnet once designated will have a gateway and a broadcast address at the bottom and top of the range. These will overlap with the larger subnet that you have created. For anything machines outside the larger subnet commuicating with machines in the smaller subnets there will have to be an appropriate router. If there is not a router for each subnet then the broadcast traffic will not be seen by most of the machines and conversely most of the machines will not be able to access the default gateway on the larger subnet. The reason I say most is that the lowest subnet broken out of the larger subnet will be able to access the gateway x.x.x.1(but not the broadcast address) and the last subnet broken out of the larger subnet will be able to access the broadcast address x.x.x.255(but not the gateway).
You may want to encourage him to try it. Its a great excersise. We just had one of our admins assign his new servers to a /24 when the network it was on was a 23. That was interesting. if you established an outbound(offnet) session with the server then you could access it from other nets just fine. But once the arp record staled in the router it would fall off line. The problem was that he was on the fist leg of the larger subnet so he could access the gateway and move traffic but when you tried to access his server from off net the broadcast was going to the x.x.x.255 on a different net.
Fun...
Hope my ramblings help!
Jeff M