Cloudflare:

On the 11th of January 1982 twenty-two computer scientists met to discuss an issue with ‘computer mail’ (now known as email). Attendees included the guy who would create Sun Microsystems, the guy who made Zork, the NTP guy, and the guy who convinced the government to pay for Unix. The problem was simple: there were 455 hosts on the ARPANET and the situation was getting out of control.

In the original ARPANET design, a central Network Information Center (NIC) was responsible for maintaining a file listing every host on the network. The file was known as the HOSTS.TXT file, similar to the /etc/hosts file on a Linux or OS X system today. Every network change would require the NIC to FTP (a protocol invented in 1971) to every host on the network, a significant load on their infrastructure.

Having a single file list every host on the Internet would, of course, not scale indefinitely. The priority was email, however, as it was the predominant addressing challenge of the day. Their ultimate conclusion was to create a hierarchical system in which you could query an external system for just the domain or set of domains you needed. In their words: “The conclusion in this area was that the current ‘user@host’ mailbox identifier should be extended to ‘[email protected]’ where ‘domain’ could be a hierarchy of domains.” And the domain was born.

Delightfully nerdy story.