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Date: Tue, 14 Mar 1995 12:26:56 -0500 (EST)
From: "Jonathan M. Bresler" <jmb@kryten.atinc.com>
Subject: Re: Routing / gateway
To: Jan_Guldemond <jg@euronet.nl>
cc: questions@FreeBSD.org
In-Reply-To: <199503141209.NAA07591@mail.euronet.nl>
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On Tue, 14 Mar 1995, Jan_Guldemond wrote:

> |--------------|  |------------------------------|  |----------------|
> | FreeBSD      |  | FreeBSD                      |  | MS-Windows     |
> | 193.78.175.1 |  | 193.78.175.2  193.78.175.129 |  | 193.78.175.138 |
> |--------------|  |------------------------------|  |----------------|
>        |                  |             |                    |
>        \------------------/             \--------------------/
> 
> The configuration of the first FreeBSD machine (snap 2-2, one ethernet card):
> ifconfig -a:
> *** ed1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 1500
> ***	inet 193.78.175.1 netmask 0xffffff80 broadcast 193.78.175.127

                                  ^^^^^^^^^^           ^^^^^^^^^^^^^^

you are trying to subnet on a single bit.  this is illegal per the rfc's.
a net admin here wanted to do the same.  here is the mail that i sent him:

	the proposal is to subnet on 1 bit.  this is a direct violation of
rfc1122.  (ftp://kryten.atinc.com/pub/rfc/rfc1122).  
	
	a subnet mask must contain at least 2 bits.  (eg 00, 01, 10, 11.  
the 00 and 11 subnets are reserved for broadcast addresses.  the 0 and 
all-ones host parts are also reserved for boardcast addresses.  so 
subnetting on 2 bits leaves 256 - ( 64 + 64 + 2 + 2 ) == 124 host 
addresses available,  62 on subnet 01 and 62 on subnet 10.  over 50% of 
the address space is lost.  subnetting on more bits improves the ratio, 
for a while)

here are some relevant parts (lightly editted): 

         3.2.1.3  Addressing: RFC-791 Section 3.2

	[deletion]

            We now summarize the important special cases for Class A, B,
            and C IP addresses, using the following notation for an IP
            address:

                { <Network-number>, <Host-number> }

            or
                { <Network-number>, <Subnet-number>, <Host-number> }

            and the notation "-1" for a field that contains all 1 bits.
            This notation is not intended to imply that the 1-bits in an
            address mask need be contiguous.

	[deletion]

 
            (d)  { <Network-number>, -1 }

                 Directed broadcast to the specified network.  It MUST
                 NOT be used as a source address.

            (e)  { <Network-number>, <Subnet-number>, -1 }

                 Directed broadcast to the specified subnet.  It MUST
                 NOT be used as a source address.

            (f)  { <Network-number>, -1, -1 }

                 Directed broadcast to all subnets of the specified
                 subnetted network.  It MUST NOT be used as a source
                 address.

	[deletion]

   !!!!!--> IP addresses are not permitted to have the value 0 or -1 for
            any of the <Host-number>, <Network-number>, or <Subnet-
            number> fields (except in the special cases listed above).
   !!!!-->  This implies that each of these fields will be at least two
            bits long.

            For further discussion of broadcast addresses, see Section
            3.3.6.

            A host MUST support the subnet extensions to IP [IP:3].  As
            a result, there will be an address mask of the form:
            {-1, -1, 0} associated with each of the host's local IP
            addresses; see Sections 3.2.2.9 and 3.3.1.1.


jmb


Jonathan M. Bresler  jmb@kryten.atinc.com	| Analysis & Technology, Inc.  
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