Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!JESSICA.STANFORD.EDU!almquist
From: almquist@JESSICA.STANFORD.EDU (Philip Almquist)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Dividing Class B number in subnets, Routing Problem
Message-ID: <8905290248.AA01952@ucbvax.Berkeley.EDU>
Date: 25 May 89 18:37:00 GMT
References: <159@pollux.kulcs.uucp>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 35

Stephan,
> In Comer's book "Internetworking with TCP/IP", there is a paragraph
> about routing with subnets (16.10 p.202) : about the routing table :
> "each entry contains one additional field that specifies the subnet mask
> used with the network in that entry:
>      (subnet mask, network addreess, next gateway)
>
> !!BUT, in a lot of implementations (UNIX BSD 4.3, SunOS, ...), there is
> no place for a subnetmask for each entry in the routing table. 
> The subnet mask is only specified for the interface.

	Hosts don't need to know anything about subnets other than the
subnet masks of any directly connected subnets.  Therefore, a subnet
mask per interface is fine for hosts (unless you run subnets with
different subnet masks on the same cable).

	Routers, on the other hand, have a lot more problems when a
network uses more than one subnet mask.  Traditionally, IP routing
protocols have not included subnet masks in routing updates, though this
is changing.  Traditionally, IP routing software has assumed that any
net has a single subnet mask.  This is also changing.

	The description in Comer's book reflects current theory, but
multiple subnet masks on a single network is an area where current
practice has not yet caught up with the theory.  It is only fairly
recently that the Internet protocol powers that be have come to
seriously address the issue.  And yes, it can work, and will be part of
future TCP/IP standards.

	However, unless you really enjoy being on the leading edge, I
would recommend that you stick to a single subnet mask for your network.
Make each subnet fairly small (7 or 8 bits of host field), and just give
out more than one subnet number to larger departments.

						Philip