[aus.comms] Can subnets be separated by another net?

m.andrews@trl.oz.au (Murray Andrews) (07/06/90)

I have a basic question about subnet routing that has probably been asked
many times but I can't locate an answer in any article sitting around
here so ....

Is it possible to route between subnets of a class B address when the
subnets are separated by another network?

For example, given the following topology:

          +---------+
          | Host C1 |
          +---------+ 192.9.200.5
               |
       --------+-----+----- Net 192.9.200 ----------+---------
                     |                              |
               +----------+ 192.9.200.1        +----------+ 192.9.200.2
               | Gate GB1 |                    | Gate GB2 |
               +----------+ 137.147.1.10       +----------+ 137.147.2.20
                    |                              |
       ------+------+-------               --------+---+------
             |                                         |
        +----------+ 137.147.1.11                 +----------+ 137.147.2.21
        | Host B11 |                              | Host B21 |
        +----------+                              +----------+


       ^     Subnet 1 of 137.147    ^     ^       Subnet 2 of 137.147     ^
       |____________________________|     |_______________________________|

Host C1, and gateways GB1 and GB2 all connect to the one network - in this
example a class C network with number 192.9.200 (don't worry - we are not
actually using this number).

Gateways GB1 and GB2 are gateways to 2 subnets of the the class B
network 137.147 with subnet mask 255.255.255.0. There is no connection
between the two subnets except via 192.9.200.

The question is does this work?

Can hosts on either subnet route to hosts on the other (e.g. B11 <-> B21)?

Can hosts on either subnet route to host C1?

Can C1 route correctly to both B11 and B21?

Can other networks gatewayed onto 192.9.200 successfully route to hosts
on the 137.147 subnets?

If it works is there anything special I have to do to the gateways?

If this works a *lot* of IP addresses can be saved.  If this doesn't work
the only alternative appears to be to get a class B address for each
subnet (since they have up to 1000-2000 PC's each).

Any help would be much appreciated.

--------------
Murray Andrews
Telecom Australia Research Labs. (m.andrews@trl.oz.au)

pte900@csc.anu.oz (Peter Elford) (07/08/90)

In article <1867@trlluna.trl.oz>, m.andrews@trl.oz.au (Murray Andrews) writes:
> I have a basic question about subnet routing that has probably been asked
> many times but I can't locate an answer in any article sitting around
> here so ....
> 
> Is it possible to route between subnets of a class B address when the
> subnets are separated by another network?

The short answer is no.
 
> For example, given the following topology:
> 
>           +---------+
>           | Host C1 |
>           +---------+ 192.9.200.5
>                |
>        --------+-----+----- Net 192.9.200 ----------+---------
>                      |                              |
>                +----------+ 192.9.200.1        +----------+ 192.9.200.2
>                | Gate GB1 |                    | Gate GB2 |
>                +----------+ 137.147.1.10       +----------+ 137.147.2.20
>                     |                              |
>        ------+------+-------               --------+---+------
>              |                                         |
>         +----------+ 137.147.1.11                 +----------+ 137.147.2.21
>         | Host B11 |                              | Host B21 |
>         +----------+                              +----------+
> 
> 
>        ^     Subnet 1 of 137.147    ^     ^       Subnet 2 of 137.147     ^
>        |____________________________|     |_______________________________|
> 
> Host C1, and gateways GB1 and GB2 all connect to the one network - in this
> example a class C network with number 192.9.200 (don't worry - we are not
> actually using this number).
> 
> Gateways GB1 and GB2 are gateways to 2 subnets of the the class B
> network 137.147 with subnet mask 255.255.255.0. There is no connection
> between the two subnets except via 192.9.200.
> 
> The question is does this work?

No. The reasons is that the Gateways will only advertise a route to 137.147.0.0
(not to a particular subnet of that network) over the 192.9.200.0 subnet they
are connected too.

> Can hosts on either subnet route to hosts on the other (e.g. B11 <-> B21)?
> 
> Can hosts on either subnet route to host C1?
> 
> Can C1 route correctly to both B11 and B21?
> 
> Can other networks gatewayed onto 192.9.200 successfully route to hosts
> on the 137.147 subnets?
> 
> If it works is there anything special I have to do to the gateways?

The only way it *might* work is to give the interfaces that connect the two
gateways a second IP address (in this case from another subnet of 137.147) 
and use a bit of static routing in the hosts. Two subnets (from different net
numbers would then share the same physical network).

cisco routers support secondary interface addresses but will never generate an
IP packet with the secondary IP address - which is why you might have to use
some static routing. I've done something like this at Macquarie University to
support CSIRO's links into that campus, but at that site it's a case of two
networks on the same cable, not a partititioned subnets (which is illegal
according to the RFC),

Peter Elford,
AARNet