>> This gives me an idea... Such a sequence of numbers is a potentially
>> valid DNS name - assuming there's a TLD called 572. If we could set one
>> up, that would MESS WITH PEOPLE'S MINDS! (...and probably reveal a bug
>> or two in the various software for parsing URLs.)
>
> Ohh, I do like the idea. But can the domain- or hostnames contain only
> numbers? Hostnames ones wasn't allowed to start with one, but AFAIK
> that's overruled now.

IETF RFCs #1034 and #1035 both contain a "preferred name syntax" 
(RFC-1034 section 3.5 and RFC-1035 section 2.3.1) which lists the 
following grammar:

   <domain> ::= <subdomain> | " "
   <subdomain> ::= <label> | <subdomain> "." <label>
   <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
   <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
   <let-dig-hyp> ::= <let-dig> | "-"
   <let-dig> ::= <letter> | <digit>
   <letter> ::= any one of the 52 alphabetic characters A through Z in
   upper case and a through z in lower case
   <digit> ::= any one of the ten digits 0 through 9

   Note that while upper and lower case letters are allowed in domain
   names, no significance is attached to the case.  That is, two names
   with the same spelling but different case are to be treated as if
   identical.

   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, RFC #1123 states (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.

It also suggests:

            If a dotted-decimal number can be entered without such
            identifying delimiters, then a full syntactic check must be
            made, because a segment of a host domain name is now allowed
            to begin with a digit and could legally be entirely numeric
            (see Section 6.1.2.4).  However, a valid host name can never
            have the dotted-decimal form #.#.#.#, since at least the
            highest-level component label will be alphabetic.

This appears to be saying essentially what I said - there is no TLD 
named 572. If there *was*, then DNS names could become ambiguous with IP 
addresses...

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