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> 

> 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 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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