I'll suggest something other than DynDNS, and actually use Perl in the process. I keep an external web server with this simple CGI:

#!/usr/bin/perl
use CGI 'header';
print header('text/plain');
print $ENV{REMOTE_ADDR};
[download]

Then you can query that CGI from inside your network and get the IP address. That information is stored on the router itself, of course, but it isn't necessarily easy to get at automatically.

Abigail (bless my ISP who provides me with a static IP address)

How do you access the web interface if you don't know its ip address?

Update: This was stupid. You mean the external ip address, I see.