Re: Subnetting, private networks & default routes

Barry Margolin wrote:
I would be happy with 172.16 being location A, and 172.17 being
location B; but unfortunately (at the moment) .16 is a mix of several
locations. I don't see what's so bad, though, if 172.16.1 and 172.16.2
are in different locations?

If the subnet mask is 255.255.0.0, then they will not send to a router
to communicate with each other, because they think they're on the same
subnet.

Okay, and therefore some of the packets will be dropped because some of
the 172.16s are actually remote... right?

Proxy-arp means that the router will respond to ARP requests for any
remote subnets. So if a machine on the LAN has its subnet mask too
large, and thinks that remote subnets are local, the router will make up
for the mistake.

Yep, I found it in the IOS manual on Cisco's site. Seeing as it
requires zero configuration it actually looks like it might be a good
fix, albeit a temporary one. Provided it does't break anything in the
process, of course...

But if I want the two nets to communicate, shouldn't the two masks be
the same? In a perfect world, anyway.

Different networks can certainly have different subnet masks.
172.16.x.x/16 means that all 172.16.*.* addresses are on the local
subnet, so 172.30.x.* are remote. 172.30.x.y/24 means all 172.30.x.*
addresses are on the local subnet, so 172.16.*.* are remote.

Right, I know that normally that's the case; but in this case where
some of them happen to be remote -- which is why I get confused. :-)

It sounds like you really need to read a tutorial on how subnet masks
are used.

It does. And, I did look up a bunch; but they were either quite too
basic or /way/ too complicated. Counting out network addresses into
binary does not interest me 8-)

A /16 subnet mask means that all addresses with the same first two
octets as yourself are on your own subnet. So machines on the 172.16.1
subnet think that the 172.16.120 machines can be reached directly,
without going through a router.

Okay. So in my case, some of the 172.16s are remote; and therefore
reachable only by routers. Which means I get the three options:
proxy-arp as a temporary fix, moving the networks around as a permanent
fix (no thanks), or changing subnets masks to /24. Correct?


Chris

.

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