what's a valid domain name?

pyh at mail.nsbeta.info pyh at mail.nsbeta.info
Sun Jan 30 12:30:08 UTC 2011

 From RFC 1123 

        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


pyh at mail.nsbeta.info writes: 

> 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.
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