My comments are below your questions with *** :
----Original Message Follows----
From: Bill Freeman <[EMAIL PROTECTED]>
CC: [EMAIL PROTECTED]
Subject: Re: supernetting ?
Date: Fri, 31 Mar 2000 16:48:58 -0500
[Discussion of increasing past subnet size and the possibility that
the address-wise adjacent subnet isn't available, and that subnet
masks must be continuous elided.]
Can any of the truly IP.aware comment on the scheme below?
IIUC, it is perfectly possible to have two separate subnets on
the same ethernet. It's just that a pair of machines on separate
subnets can't directly communicate. There needs to be a router (or
should I be saying gateway) *** they are both right*** between the subnets
to let them talk, and
the packets will pass on the ether twice: once from source to router;
once from router to destination. This is a waste, but if as
separately theorized, this is for a dorm, the vast majority of the
traffic will be going out the main gateway to the rest of the edu
domain and the internet at large. Careful partitioning of machines
across the two subnets so that any pairs that do actually communicate
a lot are on the same subnet can further reduce the unnecessary
retransmission overhead.
Certainly the router that routes between the two subnets could
have two net.cards on the same ether, but it may even be possible to
have two logical interfaces, each with their own IP address and
netmask, run through a single physical card (nothing in the hardware
to stop it).
*** This is true. We call them Secondary Addresses, or Subaddresses. This
concept is called "One Armed Routing" If you do have 2 interfaces, split up
your subnets into 2 groups it makes for a cleaner, traffic load per segment.
If you can avoid one armed routing, please do. ***
A remaining make or break question is can the gateway to
places not on the ether, which is possibly some commercial and rigidly
configured box (or under the authority of rigid and less than
brilliant network administrators) be configured to route packets for
the two disjoint subnets to the same network interface. If so, then a
bonus question is whether it could be the inter-subnet gateway.
*** Yes, all that the "commercial" gateway needs to know is to send data
destined for W.X.Y.Z and Z.Y.X.W to the next hop router interface that the
commercial router has a direct LAN/Subnet connection to. Your router will
then know where to send the packets based on its direct LAN connections, and
routing table.***
Joe Carpentier
Routing Support
Cabletron Systems
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