From: nichols@dstos3.dsto.gov.au
Newsgroups: comp.dcom.sys.cisco
Subject: IP SUBNET ANSWERS
Date: 19 Dec 92 11:55:02 CST
Organization: Defence Science and Technology Organisation

In article <527.829.uupcb@freddy.ersys.edmonton.ab.ca> murray.chapman@freddy.ersys.edmonton.ab.ca (Murray Chapman) writes:

>     I'm from the novell environment where a cable segment running IPX
>protocol takes on a net number as an ID and I'm told this is not the
>same when running IP. A call to cisco tech support told me that my first
>problem was trying to do this:

>   Me 142.60.254.1   <-- Router -->  142.60.254.2

>   when what I really needed was:

>   Me 142.60.254.1   <-- Router -->  142.60.253.1

>   My question is, does this work when using multiple subnets on one
>side of the router as follows or does the port on the router need an
>address in each subnet that you want to route?

>    Me 142.60.254.1  <-- Router --> 142.60.253.1 - 254
>                                    142.60.252.1 - 254
>                                    142.60.001.1 - 254

To answer the question one can consider that the situation can be interpreted
two ways. So that either proposition can be true. The uncertainty is in what
is meant by "multiple subnets on one side of the router".

(Throughout this posting I assume that the network is subnetted with a
ffffff00 mask. I infered that from the addresses in the example).

That could mean a number of conventional IP subnets; each on it's own
cable and seperated by a multiport router or series of routers. In which case
the first proposition is true. Only the subnets which are directly connected
to the router must be specified in the attatched interface's address. Any other
subnets available through other routers will be learnt by the routing protocol.

Alternately it could mean that you want to configure three IP subnets to
exist on the same cable; so that all three are logically directly connected
to the one port. This is only possible by the use of a special feature of the
ciscos. They allow multiple addresses (which identify multiple subnets) on one
interface; primary (normal) and additional single or multiple secondary
addresses. All routers connected to this network must support this feature
and their interfaces must also have an address in each of the secondary
subnets. Otherwise more complicated counter measures to correct the routing
would be needed.

You can save yourself from unneccessarily complex problems if you forget this
difference between IPX and cisco's IP and forget secondary addresses and
multiple subnets; unless you must use them. The definition of an IP subnet
is normally the same as an IPX network; except for this cisco feature.
The cisco manual describes the intended applications of secondary addresses.
They include - when you want more than 254 IP nodes on one physical subnet.
             - when migrating from an unsubnetted IP network to a subnetted one.

We have used the feature for the later reason but are attempting to
progressively remove all secondaries. We want to have a more conventional
IP network. This will simplify debugging by removing uncertainty about
interworking with non-cisco products, ie. we needed static routes to Apollo
routers.

Regards,  Peter.
Newsgroups: comp.dcom.sys.cisco