How do I subnet a class B network

[Maj Brian Boyter]

jmorris at mwunix.mitre.org (John R. Morris) writes:
>I have some 4/110s and one 4/280 which were on a class B network.  The
>4/280 now has two ethernet interfaces and I want to make it a router,

>On the 4/280 (running Sun OS 4.1) can I do the following:?
>	ie0 (rest of world, Class B)
>		inet 128.29.22.1 netmask 255.255.0.0 broadcast 128.29.255.255
>	ie1 (my little corner, Class C?)
>		inet 128.29.22.7 netmask 255.255.255.0 broadcast 128.29.22.255


John, I was holding up on answering your query, waiting for someone who is
a true guru to come to the rescue...   But I haven't seen an answer posted
by anyone else...  Anyway, here's my idea of what a "normal" Class-B
network would look like:

            |
       -----------
       | gateway | (e.g. gateway to nsfnet)
       -----------
            |128.29.22.1
            |                     backbone
 _____________________________________________________
     |                    |                   |
     |subnet#1            |subnet#2           |subnet#3
     |                    |                   |
     |                    |                   |
     |128.29.22.2         |128.29.22.3        |128.29.22.4
 --------             --------            --------
 |router|             |router|            |router|
 --------             --------            --------
     |128.29.23.1         |128.29.24.1        |128.29.25.1
     |                    |                   |
     | ------             | ------            | ------
     |-|host|             |-|host|            |-|host|
     | ------             | ------            | ------
     | 128.29.23.2        | 128.29.24.2       | 128.29.25.2
     |                    |                   |

I would use a netmask of 255.255.255.0 on both sides of the routers...  If
the routers are running routed, or some other routing algorithm, then the
routers will learn the routes to the other subnets...    If not, you can
manually install routes to the other subnets using route(8).  I would put
the these manual routes in the rc.local...  For example, if the gateway
didn't talk the right routing protocol to the router, you could just put:
route add default 128.29.22.1 2 in the rc.local to provide that missing
route....

 I hope this helps,
 Brian

   Maj. Brian A Boyter

   boyter at fstc-chville.army.mil                      ||