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

Kurt Zeilenga (hi!
Mon, 27 Jul 87 11:42:49 MDT writes:
> I'd like to understand the reason that you feel the need to split
> a class B network into different-sized subnets. What happens if
> you stick to a single subnet size?

The reasons why we would like to split the B into
different-sized subnets is simple. Address space. Right
now we are projecting to have over 1k hosts on our main
ethernet (actually a combination of thin and fat wires
connected together using repeaters and smart bridges, like
DEC's LAN 100 bridge). We also have many subnet that are
being installed. Some of which are very small. To
accomadate the main cable we would have to use a mask like
0xfffff800. This means we have split our B into 32 subnets
each of 2k hosts (minus all ones and all zeros, of
course). Anyway, 32 subnets will probably not be enough.
Most (if not all) of our subnets will be gatewayed directly
to the main ethernet (so subnetting the subnet won't really
come into the problem)

> Although some of the comments in reply to your message have been somewhat
> overblown, the fact is that the technical mechanism to handle
> different-sized subnets of the same network is not generally available
> today. It may require carrying a 32-bit subnet mask along with each IP
> (sub-)network address in whatever IGP is used within the subnetted
> network. The only current IGP which does carry such a mask is Dave
> Mills' Hello protocol used in the Fuzzballs; however, you could probably
> hack the BSD routing table and daemon to do so. If you are not in a
> position to roll your own IGP in this fashion, you had better stick to a
> single subnet mask.

We are not planning to do any hacking. Since there is no
real software solution at this time (unless you hack),
maybe a hardware solution is in order. Anyone know of
where we could pick up a few "smart bridges" real cheap?

> Bob Braden

        - Kurt (

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