On Thu, 14 Jul 2005, Robert A. Rosenberg wrote:
At 14:51 +0100 on 07/13/2005, willemien(_at_)amidatrust(_dot_)com wrote about Re:
draft-klensin-rfc2821bis-00.txt structural and textual :
section 2.3.10 has a "fuzzy" reference
"the host specified in the domain part of the address."
in general the host is NOT the domain part so what is meant by the host
specified in the domain part of the address??
The address is of the form UserID(_at_)Domain (with the "Domain Part" being
everything to the right of the @-sign). The Domain part has a host if there
are more levels than the number defined for the WhoIs registration. If the
domain is example.com but the Domain in the address is X.SMTP.EXAMPLE.COM
then X is the host in the SMTP subdomain of domain EXAMPLE.COM.
While "X is a host of SMTP subdomain of domain example.com" is correct,
so would it be that "X is a host of subdomain SMTP.EXAMPLE of domain .com"
or that "X host of domain SMTP.example.com" or for that matter that
X.smtp.example.com is fully qualified host name.
Even though certain levels of delegations exist in ICANN tree and can
sometimes be found based on whois - these are not technical levels and
you can not make an assumption that some part is a domain, something
is subdomain and something is a host just by looking at the dns name.
Based on dns terms in reality every hostname is a domain on its own
(subject to that there is some difference in characters allowed in
hostnames and domains, but that rarely comes into play).