This manual page describes the format of the /etc/hosts file. This file
is a simple text file that associates IP addresses with hostnames, one line
per IP address. For each host a single line should be present with the
IP_address canonical_hostname aliases
Fields of the entry are separated by any number of blanks and/or tab characters.
Text from a "#" character until the end of the line is a comment,
and is ignored. Host names may contain only alphanumeric characters, minus
signs ("-"), and periods ("."). They must begin with an
alphabetic character and end with an alphanumeric character. Aliases provide
for name changes, alternate spellings, shorter hostnames, or generic hostnames
(for example, localhost). The format of the host table is described in
The Berkeley Internet Name Domain (BIND) Server implements the Internet name
server for UNIX systems. It augments or replaces the /etc/hosts file or
host name lookup, and frees a host from relying on /etc/hosts being up
to date and complete.
In modern systems, even though the host table has been superseded by DNS, it is
still widely used for:
Most systems have a small host table containing the name
and address information for important hosts on the local network. This is
useful when DNS is not running, for example during system bootup.
Sites that use NIS use the host table as input to the NIS
host database. Even though NIS can be used with DNS, most NIS sites still
use the host table with an entry for all local hosts as a backup.
Very small sites that are isolated from the network use the
host table instead of DNS. If the local information rarely changes, and
the network is not connected to the Internet, DNS offers little
Before the advent of DNS, the host table was the only way of resolving hostnames
on the fledgling Internet. Indeed, this file could be created from the
official host data base maintained at the Network Information Control Center
(NIC), though local changes were often required to bring it up to date
regarding unofficial aliases and/or unknown hosts. The NIC no longer maintains
the hosts.txt files, though looking around at the time of writing (circa
2000), there are historical hosts.txt files on the WWW. I just found three,
from 92, 94, and 95.