what's a valid domain name?

[pyh at mail.nsbeta.info]

Joseph S D Yao writes: 

> 
> 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. 
> 

A label must start with a letter? oh I don't think so.
How about these domains which all have huge DNS traffic? 

163.com
126.com
51.com
56.com 

yes 163.com is a domain name but "163" also can be treated as a label for 
domain "com.", is it? 

Thanks. 

Regards.