port forwarding

[Scot]

Not sure if this can even be done at the dns server or not.  Want to port 
forward to a different port for a web server.

So for example, www.example.com is hosted on port 81.  When someone looks up 
www.example.com, I want to redirect them to www.example.com:81 or something 
along those lines.  I know certain services like no-ip.com use something 
called web forwarding.  Wasn't sure if they were doing this via dns records, 
or if they are sending them first to a dummy website they host - then 
redirecting via a http header or something to your host with the new port.

Can anyone let me know of a way of doing this via dns if possible?

Thanks,

Scott