Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!hub.ucsb.edu!spectrum.CMC.COM!lars
From: lars@spectrum.CMC.COM (Lars Poulsen)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Subnetting a very large population
Keywords: subnetting, rfc950, rfc1219, tcp, ip
Message-ID: <1991Jun14.065151.19238@spectrum.CMC.COM>
Date: 14 Jun 91 06:51:51 GMT
References: <m1TF42w163w@sadss>
Organization: CMC (a Rockwell Company), Santa Barbara, California, USA
Lines: 48

In article <m1TF42w163w@sadss> garethh@sadss (Gareth Howell ) writes:
>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 am involved with defining a similar network in this country.
Ours is worse, in that most of the X.25 connections are dial-up
on-demand (inbound-only).

The proposed solution in our network is to hierachically define all of
the address space in a number of class B networks, one per region, with
the physical X.25 WAN appearing in segments of each class B net.
The regions are each headed up by an IP router, and these routers
(which will probably all be co-located with a major hub in the
X.25 network) will be connected via a backbone LAN.

Each leaf PC only needs a default route to the Ether side of the leaf
gateway. Each leaf gateway only needs a default route via X.25 to the
regional router. The regional router needs to know the subnet numbers,
subnet masks and X.25 addresses of all the ethernet segments within the
region. But anything outside the region can be routed by net number
(without subnets).

To make this work, an IGP must be used which can communicate masks with
all routes, but the whole cluster can connect to the outside world by
EGP and only tell the core about the regional class B numbers.

Note, that things would have been defined very differently if the
wide area network had been managed primarily for IP use and had had an
established DDN-like IP-to-X.25 address mapping.

I hope this is helpful. Feel free to contact me by email. The above is
probably as much as is useful to the world.
-- 
/ Lars Poulsen, SMTS Software Engineer
  CMC Rockwell  lars@CMC.COM