Before DNS existed, host-to-IP mapping was maintained in a single flat file — HOSTS.TXT — distributed by the Stanford Research Institute and downloaded periodically by every machine on the ARPANET. By the early 1980s the network had grown large enough that this arrangement was obviously doomed, and Paul Mockapetris designed DNS in 1983 to replace it. The flat file survived as /etc/hosts, consulted before DNS on most systems, used for local overrides, development environments, and ad-blocking by people who populate it with every known advertising domain. Every operating system — Linux, macOS, Windows — still ships with /etc/hosts and still checks it first. It is consulted billions of times a day. The SRI stopped distributing HOSTS.TXT in 1990. The file it spawned will probably outlive DNS itself.
