How To Find My Previous IP Addresses

Also in Gmail, at the bottom right is a link to your account activity.

Clicking it will show information, including the accessing IP address, about the last 10 Gmail sessions:

Depending on how you send e-mail, your current IP (at that time) might be present in the Received: header of mails other people got from you.

Try and send an e-mail to yourself and look at its source code (how to do that depends on how you access your mail) and compare the addresses that show up in the various Received: headers to see whether your current address appears there.

If it does, you “just” have to somehow get one of the e-mails you sent at the time you’re interested in. You might have sent one to a friend or to a mailing list.

(Yes, the chances to find the IP this way are pretty low, but depending on how important it is to find it out it might be worth the effort.)

Your modem/router might keep logs on which IP the DSL was registered to. Usually the modem's logs/UI is accessible by navigating to its IP through your browser.