Internet URLs, file system paths and IP addresses usually get more specific as you move towards the right hand side. This is a clearly a byproduct of the left-to-right language of the designers of these systems. So why is it that this website is david.newgas.net and not net.newgas.david?

The familiar hierarchical naming is part of the “Domain Name System” created in the early 80’s by early internet engineers. They already had a internet naming system with cryptic, non hierarchical names like BBN-TENEXD and MIT-Multics . This was basically maintained in one giant text file managed by one organisation.

Faced with a network growing at ever increasing pace this system was becoming a nightmare to maintain. A naming system that followed administrative hierarchies seemed obvious, and was spelt out in RFC 822.

At this stage the right-to-left notation was already settled so to answer our question we must go further back to RFC 805, with the unhelpful title “Computer Mail Meeting Notes”. Here they discuss exactly how to add the extra hierarchical info.

One of their biggest concerns was email addresses. These were already of the form user@host and they wanted something that extended this naturally. Here were some options they may have thought of:

Cerf@ISI → Cerf@ARPA.ISI

ARPA.CERF@ISI

Cerf@ISI.ARPA

It’s fairly easy to see why they picked Cerf@ISI.ARPA – the information consistently gets less specific as you go along. The other options are more jumbled. Quoting the RFC makes me sound foolish for even thinking the addresses are backwards:

… the [email address] would read (left to right) from the most specific to the most general.

So there you have it – domains have the most specific part on the left hand side to make email addresses look good!