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From: JOHN@heap.cisco.com (John Wright)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Subnet 0
Message-ID: <12641958114014@heap.cisco.com>
Date: 30 Nov 90 05:49:59 GMT
References: <1120@soleil.UUCP> <1990Nov27.025444.15666@bwdls61.bnr.ca> <20599@crg5.UUCP>
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Ed,

To use your example of class B 138.95.0.0 with subnet mask 255.255.255.128
or 138.95.0.0 with 9 bits of subnetting.

A subnet of zero is any host address with the subnetting bits set to
zero, or rather hosts 138.95.0.1 thru 138.95.0.126. All of these
addresses are on 'subnet zero'. The address 138.95.0.0 is not a host
address for obvious reasons, the address 138.95.0.127 is not because
with a subnet mask of 9 bits this is a broadcast address to subnet zero,
when using the all ones form of the broadcast.

Let's concentrate most significant two octets of the internet address
(as 138.95 won't change) our subnet mask of 255.128 is:
11111111 10000000

138.95.0.1 would have right two octets 00000000 00000001

the first normal subnet in this case would be 00000000 10000000 the first
real host then would be 00000000 10000001 or 138.95.0.129. The address
138.95.0.255 with nine bits subnetting becomes a directed broadcast at the
128 subnet or  00000000 1 1111111  
                        ^ ^^^^^^^
                   subnet   host

See the old gateway manual chapter 11, the 8.1 gateway manual chapter 5,
Douglas Comer's book on TCP/IP, or RFC 950 for more details.

John Wright
Customer Engineering
cisco Systems, Inc.