The hosts file contains information regarding the known hosts on the network. It can be used in conjunction with DNS, and the NIS maps hosts.byaddr and hosts.byname, as controlled by nsswitch.conf(5). For each host a single line should be present with the following information:
official host name
Items are separated by any number of blanks and/or tab characters. A # indicates the beginning of a comment; characters up to the end of the line are not interpreted by routines which search the file.
When using the name server named(8), this file provides a backup used when the name server is not running. For the name server, it is suggested that only a few addresses be included in this file. These include addresses for the local interfaces that ifconfig(8) needs at boot time and a few machines on the local network.
This file may be created from the official host data base maintained at the Network Information Control Center (NIC), though local changes may be required to bring it up to date regarding unofficial aliases and/or unknown hosts. As the data base maintained at NIC is incomplete, use of the name server is recommended for sites on the DARPA Internet.
Network addresses are specified in the conventional . (dot) notation using the inet_addr(3) routine from the Internet address manipulation library, inet(3). Host names may contain any printable character other than a field delimiter, newline, or comment character.