DNS for IP address lookup

Barry Margolin barmar at alum.mit.edu
Wed Jan 12 05:58:21 UTC 2005


In article <crvl3q$2o6j$1 at sf1.isc.org>, Danny Mayer <mayer at gis.net> 
wrote:

> At 04:14 PM 1/10/2005, John Smith wrote:
> >since nobody else responded, i will.
> >if you type in an IP address anywhere for network connectivity, eg web
> >browser, command prompt, even a news reader, no DNS (or WINS
> >depending on situation) lookups are needed or performed - unless you have
> >some sort of logging or some other feature that requires reverse lookups
> >(ip address to name lookup).
> 
> If only that were true. There's plenty of badly written software out there
> that will attempt to do lookups even though there's no need.
> Just ask the root operators about the traffic that they see.

How is it supposed to know there's no need?  For all it knows, when you 
type 1.2.3.4 you actually meant a host in the .4 top-level domain.  A 
reasonable algorithm is to first try the given string as a hostname; if 
that fails, and it looks like a dotted quad, parse it as an IP address.

Yes, it's true that there's no such TLD.  And there's also a rule that 
says that labels can't be all-numeric.  But rules like that are subject 
to change, so it can be a bad idea to hard-code them into applications 
(once upon a time the rule was that the first letter of a label had to 
be a letter -- the rule was relaxed when 3Com got onto the Internet and 
was allowed to register 3COM.COM).  The algorithm I describe will 
automatically adapt to current conventions.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***



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