Host file - Definition and Overview

In computing, a host file, stored on the computer's filesystem, is used to look up the Internet Protocol address of a device connected to a computer network. The host file describes a many-to-one mapping of device names to IP addresses. When accessing a device by name, the networking system will attempt to locate the name within the host file if it exists. Typically, this is used as a first means of locating the address of a system, before accessing the Internet domain name system. The reason for this is that the host file is stored on the computer itself and does not require any network access to be used, whereas DNS requires access to an external system, which is typically slower.

Location

The host file is located in the following directories for each operating system:

• Linux - /etc
• Windows 95/Windows 98/Windows Me - C:\windows\
• Windows 2000/Windows XP - C:\windows\system32\drivers\etc\
• Windows NT - C:\winnt\system32\drivers\etc\

Ad filtering

One useful ability of the host file is that it is capable of ad filtering. This is accomplished by adding a line to the file that contains 127.0.0.1 (home IP) or 0.0.0.0 (no IP). That way, when a internet capable program attempts to access the IP of an advertiser the ad is prevented from appearing.

For example, if you wanted to block Doubleclick ads, you could ad the following to your host file:

127.0.0.1 ad.doubleclick.net

or

0.0.0.0 ad.doubleclick.net