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From: garethh@sadss (Gareth Howell )
Newsgroups: comp.protocols.tcp-ip
Subject: Subnetting a very large population
Keywords: subnetting, rfc950, rfc1219, tcp, ip
Message-ID: <m1TF42w163w@sadss>
Date: 12 Jun 91 16:51:33 GMT
Organization: Department of Social Security
Lines: 36

Hi all,
I wonder if someone can help me with a tricky problem I am trying to come to 
grips with.

I have the (un)enviable taks of coming up with an Internet Addressing 
Strategy for the UK Department of Social Security's internet (note small 
'i':-). This comprises (or soon will) 2500+ Ethernet LANs: each with 
anything from 4-50 PCs, Unix application servers and gateways on them; all 
interconnected using the Government Data Network (X.25 (1980)). Most of the 
operational systems use OSI protocols, but there is a significant amount of 
IP traffic, mainly for SNMP HUB and BRIDGE and host management on the LANs.

What I need is a sanitory way to split up the population to ease number 
allocation and permit local administration of each LAN. What I have come up 
with is this, and I would like comments (good or bad :-):

Allocate a single non-subnetted Class B address to the X.25 GDN (2500+ hosts).
Allocate a number of Class B addresses to clusters of LANs, and subnet each 
of these networks in accordance with RFC950 and RFC1219.

I have one outstanding issue relating to this, and that is whether dynamic 
routing protocols will cope with this environment. Specifically, will a host 
on LAN 'A' (which is a subnet of network 'X') be able to reach a host on LAN 
'B' (which is a subnet of network 'Y') by routing across the GDN. The 
problem seems to be whether the routing tables in LAN 'A's GDN gateway, 
know that to get to LAN 'B' you have to go to LAN 'B's GDN gateway: which 
implies that LAN 'A's gateway (on network 'X'), knows the subnet mask of 
network 'Y'. I'm not sure this is possible; but the alternatives of 
allocating a single Class A network address to cover the lot, or allocate 
2500+ Class C addresses + 1 Class B address (for the GDN) are either 
impractical, or unmanageable (and anti-social if we advertise 2500+ networks 
to the core!!! ).

Any ideas?
Gareth Howell
garethh%sadss.uucp@ukc.ac.uk (I think that's the best route for mail)