Why did hostnames get reversed?

The earliest RFC which I know of which spells it out is RFC 805, <a href="https://tools.ietf.org/html/rfc805" rel="nofollow">https://tools.ietf.org/html/rfc805</a><p><pre><code> It was clear that the information content of all these syntactic alternatives was the same, so that the one causing least difficulty for existing systems should be chosen. Hence it was decided to add all new information on the right of the @ sign, leaving the "user" field to the left completely to each system to determine (in particular to avoid the problem that some systems already use dot (.) internally as part of user names). The conclusion in this area was that the current "user@host" mailbox identifier should be extended to "user@host.domain" where "domain" could be a hierarchy of domains. In particular, the "host" field would become a "location" field and the structure would read (left to right) from the most specific to the most general. </code></pre> It was chosen apparently because RFC 805 was standardizing to "mailbox@computer", and since 'mailbox' is more specific than 'computer' it became more and more specialized going left.<p>As for why 'mailbox' comes first, it's presumably because that's someone's username. The very first thing you want to see is, "I'm sending this to drostie," and only afterwards do you need to know where that server is located.

I don't have any concrete evidence, but my guess is that having the repeating value (the TLD) at the front meant that storing domain names in a hashtable was inefficient. Lookups would be faster if the more-unique part was at the front of the string, since the string comparisons would run faster.