Re: Net Nums and Gateways

Mike Brescia (brescia@PARK-STREET.BBN.COM)
Wed, 06 Apr 88 08:41:14 -0400

     <from STJOHNS>
     You are doing exactly the right thing... 42 class B addresses.

Since the idea of subnetting is to hide the distinction among the subnets from
the rest of the routers, you can use a single net which you subnet. The
penalty you pay is to have the other routers send to an arbitrary one of your
42 gateways in order to reach a particular subnet. This means you must be
prepared to take a packet addressed to subnet #27 in on the gateway for subnet
#33 and forward it to the proper your other gateway for subnet #33. You may
have better (faster) connections inside your domain than on the DDN, so it may
not be too painful in user delay or DDN overload.

If the source of the packet is a host on the DDN, rather than a gateway, then
your gateway can send an ICMP redirect-host to get further packets to the
proper gateway [flames about hosts which do not listen to redirect-host go to
those hosts].

This example has come up often in the past, but usually the number of sites
has been around 2 rather than 42. For the cases where the sites have no
separate connection, they used separate nets. For the cases where the
sites had backdoor trunks, or a bridged ethernet, they used a single net, and
paid the extra penalty of sending traffic over their internal trunk that came
in on the 'wrong' gateway.


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