numeric subdomain

Solution 1:

Yes numeric domain names are supported by just about everything. The only pitfall I can think of is it's more easily confused by end users.

Solution 2:

There are no ramifications apart from some possible ambiguity. I've done it a few times to store historical versions of sites that I've worked on.

RFC1034 was written in 1987 and superseded by, at the very least, RFC1123 (dated 1989) which includes in this update to section 2.1: The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.

The people at 37signals.com will be pleased to know that their domain is valid.

Solution 3:

Oops, ignore my answer, it appears to be outdated. See hellodaniel's answer.

Great question, I was just Googling this too. According to the official DNS standard (RFC 1034)...

The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphen. There are also some restrictions on the length. Labels must be 63 characters or less.

However, I would expect a numeric subdomain to work fine in 90-95% of apps and browsers out there. But just be aware that there may be the odd strict or unforgiving application that refuses it.