hosts file

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hosts file

(networking)
A text file on a networked computer that was used to associate host names with IP addresses. A hosts file contains lines consisting of whitespace-separated fields giving an IP address followed by list of host names or aliases associated with that address. The name resolution library software can use this file to look up the IP address for a host name. The hosts file is "/etc/hosts" on Unix and "C:\Windows\System32\drivers\etc\hosts" or "lmhosts" on Microsoft Windows,

Hosts files have now been almost entirely replaced by DNS, in which distributed servers provide the same information. A hosts file can still be used to override DNS for testing purposes or other special situations.

HOSTS file

A text file in a TCP/IP network that specifies the IP addresses of computer names and domain names. It is used to convert a request by name to a numeric IP address on the local network or the Internet.

Testing and Quick Retrieval
HOSTS files are used to override the DNS system for testing purposes so that a Web browser or other application can be redirected to a specific IP address. In addition, by explicitly stating an IP address in a HOSTS file, a Web site can be retrieved more quickly, because the multiple transmit/receive steps a normal DNS query takes have been eliminated. See DNS.

Denying Access to Web Sites
Web sites can be denied access from a computer if the URLs of those sites are added to the HOSTS file and directed to the loopback address of 127.0.0.1. For example, the following entry in a HOSTS file prevents users from reaching YouTube or eBay.

  127.0.0.1   www.youtube.com
  127.0.0.1   www.ebay.com


HOSTS File Locations
In Windows, the HOSTS file is in the \windows\system32\drivers\etc folder. In the Mac, it is in /private/etc. The first line of a HOSTS file contains the following. See localhost.

  127.0.0.1   localhost