ARD can access computers on other networks, but often it can be a headache to set up and maintain the connection. If you just need to be able to log into a client's computer, I'd suggest a service such as LogMeIn. You can use that to take control of a remote computer and it's a lot easier to set up and maintain.
Thanks so much. I couldn't tell by reading all the literature if it was possible or not and no one at Apple could help. I had a feeling it wasn't going to work the way I wanted it to. I will check out LogMeIn and see if that suits better.
Appreciate very much your responding to me so quickly.