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The hosts file ("hosts.txt") is a plain-text file that contains a list of host names and their corresponding IP addresses. It is essentially a database of domain names that is used by the operating system for identifying and locating a host in an IP network.
The forerunner of the Internet, the ARPANEt, did not have a distributed domain name database since the number of hosts then was very few. But as the popularity of the Internet increased, the idea of the hosts.txt file emerged because of the need to gather in a convenient place all the information about the different hosts in a network.
The format specification of the hosts file is described in RFC 952. The hosts file consists of one or more lines of text, where each line begins with the IP address followed by one or more host names separated by a space. The file may also contain comments, which begin with a hash sign (#); blank lines are ignored.
The rigid and centralized nature of the hosts file proved problematic, however; to address this issue, the distributed Domain Name System was created.