Hosts file

The hosts file is a computer file used in an operating system to map hostnames to IP addresses. This method is one of several system facilities to address network nodes on a computer network. On some operating systems, the host file content is used preferentially over other methods, such as the Domain Name System (DNS), but many systems implement name service switches to provide customization. Unlike DNS, the hosts file is under the control of the local computer's administrator

The ARPANET, the predecessor of the Internet, had no distributed host name database, such as the modern Domain Name System for retrieving a host's network node address. Each network node maintained its own map of the network nodes as needed and assigned them names that were memorable to the system's users. There was no method for ensuring that all references to a given node on a network were using the same name, nor was there a way to read some other system's hosts file to automatically obtain a copy.

The small size of the ARPANET made the use of hosts files practical. Network nodes typically had one address and could have many names. As individual TCP/IP computer networks gained popularity, however, the maintenance of the hosts file became a larger burden on system administrators as networks and network nodes were being added to the system with increasing frequency.

Standardization efforts, such as the format specification of the file HOSTS.TXT in RFC 952, and distribution protocols, e.g., the hostname server described in RFC 953, helped with these problems, but the centralized and monolithic nature of host files eventually necessitated the creation of the distributed Domain Name System.