.\"	$OpenBSD: networks.5,v 1.14 2007/05/31 19:19:58 jmc Exp $
.\"	$NetBSD: networks.5,v 1.9 1999/03/17 20:19:47 garbled Exp $
.\"
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.\"     @(#)networks.5	8.1 (Berkeley) 6/5/93
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt NETWORKS 5
.Os
.Sh NAME
.Nm networks
.Nd Internet Protocol network name database
.Sh DESCRIPTION
The
.Nm
file is used as a local source to translate between Internet Protocol (IP)
network addresses and network names (and vice versa).
It can be used in conjunction with the Domain Name System (DNS).
.Pp
While the
.Nm
file was originally intended to be an exhaustive list of all IP
networks that the local host could communicate with, distribution
and update of such a list for the world-wide
.Tn Internet
(or, indeed, for any large "enterprise" network) has proven to be
prohibitive, so the Domain Name System is used instead, except as noted.
.Pp
For each IP network, a single line should be present with the following
information:
.Bd -unfilled -offset indent
official network name
ip network number
aliases
.Ed
.Pp
Items are separated by any number of blanks and/or tab characters.
.Pp
A hash mark
.Pq Ql #
indicates the beginning of a comment; subsequent characters up to the end of
the line are not interpreted by routines which search the file.
.Pp
Network number may be specified in the conventional
.Ql \&.
(dot) notation using the
.Xr inet_network 3
routine from the IP address manipulation library,
.Xr inet 3 .
Network names may contain
.Qq a
through
.Qq z ,
zero through nine, and dash
.Pq Ql \&- .
.Pp
IP network numbers on the Internet
are generally assigned to a site by its Internet Service Provider (ISP),
who, in turn, get network address space assigned to them by one of
the regional Internet Registries (e.g., ARIN, RIPE NCC, APNIC).
These registries, in turn, answer to the Internet Assigned Numbers
Authority (IANA).
.Pp
If a site changes its ISP from one to another, it will generally
be required to change all its assigned IP addresses as part of the
conversion; that is, return the previous network numbers to the previous ISP
and assign addresses to its hosts from IP network address space given by the
new ISP.
Thus, it is best for a savvy network manager to configure his
hosts for easy renumbering, to preserve his ability to easily change his
ISP should the need arise.
.Sh FILES
.Bl -tag -width /etc/networks -compact
.It Pa /etc/networks
.El
.Sh SEE ALSO
.Xr getnetent 3 ,
.Xr resolver 3 ,
.Xr resolv.conf 5 ,
.Xr hostname 7 ,
.Xr named 8
.Rs
.%R RFC 2317
.%D March 1998
.%T "Classless IN-ADDR.ARPA delegation"
.Re
.Rs
.%R RFC 1918
.%D February 1996
.%T "Address Allocation for Private Internets"
.Re
.Rs
.%R RFC 1627
.%D July 1994
.%T "Network 10 Considered Harmful"
.Re
.Rs
.%R RFC 1519
.%D September 1993
.%T "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy"
.Re
.Rs
.%R RFC 1101
.%D April 1989
.%T "DNS Encoding of Network Names and Other Types"
.Re
.Sh HISTORY
The
.Nm
file format appeared in
.Bx 4.2 .