Find the WAN adress for a customer (Linux console)

view story

http://serverfault.com – I'm trying to problem solve a network Connection for a customer, and have to do a speed test on the network from Our Linux console. I'm not sure how I'll get the right WAN address. I have the gw of the router, and the ip of the network for the ISP. Is it one of these ip that I should take a traceroute on? Thanks :) (HowTos)