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NETWORKS(5) File Formats Manual NETWORKS(5)


networksInternet Protocol network name database


The networks 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).
While the networks 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 Internet (or, indeed, for any large "enterprise" network) has proven to be prohibitive, so the Domain Name System is used instead, except as noted.
For each IP network, a single line should be present with the following information:
official network name 
ip network number 
Items are separated by any number of blanks and/or tab characters.
A hash mark (‘#’) indicates the beginning of a comment; subsequent characters up to the end of the line are not interpreted by routines which search the file.
Network number may be specified in the conventional ‘.’ (dot) notation using the inet_addr(3) routine from the IP address manipulation library. Network names may contain “a” through “z”, zero through nine, and dash (‘-’).
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).
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.




getnetent(3), resolver(3), resolv.conf(5), hostname(7), nsd(8), unbound(8)


P. Mockapetris, DNS Encoding of Network Names and Other Types, RFC 1101, April 1989.
Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear, Address Allocation for Private Internets, RFC 1918, February 1996.
H. Eidnes, G. de Groot, and P. Vixie, Classless IN-ADDR.ARPA delegation, RFC 2317, March 1998.
V. Fuller and T. Li, Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, RFC 4632, August 2006.


The networks file format appeared in 4.2BSD.
August 23, 2014 OpenBSD-current