knowing my IP address behind NAT firewall

[dexter2 <dexter2@nextra.sk>]

Hi,
i'm connected to internet via DSL line. I have DSL modem with 4 eth
ports. DSL modem serve also as NAT firewall.
Each time i connect to internet, i have diferent IP, that my ISP assign
to me. If i want to know my IP, that i use on internet, I have to
connect to DSL modem via HTTP and read it there. 
Is there a unix command, that will get my internet IP? How does this
command get this IP?
It must be posible, becouse Valknut is able to get my actual internet
IP.
  Thanks
     Dexter2