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From: news@m2xenix.psg.com (Randy Bush)
Newsgroups: comp.protocols.tcp-ip
Subject: Help designing address allocation in a metronet
Message-ID: <1991Mar20.150615.18929@m2xenix.psg.com>
Date: 20 Mar 91 15:06:15 GMT
Organization: Pacific Systems Group, Portland Oregon US
Lines: 55

We have a metronet here in the Portland area, RAINet.  We are using SLIP (often
with PC-Route), and most sites have multiple hosts on a local ether.

With one exception (a private C), we are all sharing a single class C using a
subnet mask of 0xf8.  As each of the SLIP links must be a subnet, we're chewing
up our class C rather quickly.  So, we seek advice on how to best regroup and
reorganize.

We have read the paper by Tsuchiya, "On the Assignment of Subnet Numbers",
which describes a scheme for managing a net in which subnet masks differ in
length.  But are confused by the following question.  If all the neighbors of a
router have a subnet mask <= n bits, how does this router send to a distant
site whose subnet mask is > n bits.  To elaborate (thanks to Alan Batie):

   Suppose you have Subnets A (1001), B (010), C (110), D (001) and E (1000).
       A <-> B <-> C <-> D <-> E
   All three of C's interfaces (link to B, link to D and local network
   interface) have a subnet mask of 11100000.  My theory is that C will
   send all packets for subnet addresses 100 to E, except for packets for
   a host "10010000", which won't exist, because that is A's "this network"
   address.

   To be fair, he does say that you need support for varying subnet masks,
   and specifically mentions OSPF, which does propagate the subnet mask as
   well as the address.

In general, we are unsure whether the varied software (PC-Route, SysV/386,
Xenix, NeXT, VMS/TGV) in RAINet will be able to handle varying subnet masks.

We also have another concern.  As many of the inter-site links are or will be
V.32 or so, we are worried that RIP will eat up bandwidth.

Some of the alternatives we see are:

  o get a class B and subnet with a mask of 0xfff0, with SLIP links still
    eating up a subnet apiece,

  o get a class B and subnet with a mask of 0xff00, but keep the current
    class C for the SLIP links using a subnet mask of 0xfc,

  o each site get its own class C (even if they have but two hosts), and use
    the current C for the SLIP links using a subnet mask of 0xfc, thereby
    creating a gawdawful lot of underutilized class Cs,

  o get n class Cs, where n is, for example, sites/5, and clump multiple sites
    in each C according to physical topology in order to cut down RIP use of
    the small bandwidth, and

  o we're sure there are others.

We presume that we're missing something obvious, that others have gone before
us, and so await your sagely advice.

-- 
Randy Bush  / news@psg.com  / ..!uunet!m2xenix!news