Date: Tue, 14 Mar 1995 12:26:56 -0500 (EST) From: "Jonathan M. Bresler" <jmb@kryten.atinc.com> To: Jan_Guldemond <jg@euronet.nl> Cc: questions@FreeBSD.org Subject: Re: Routing / gateway Message-ID: <Pine.3.89.9503141219.D9876-0100000@kryten.atinc.com> 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.
| 2341 Jeff Davis Hwy
play go. | Arlington, VA 22202
ride bike. hack FreeBSD.--ah the good life | 703-418-2800 x346
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