The Signup IPs addon posts an IP address for all signups through any Email Marketing form.
The IP address is also captured for all confirmation clicks. This is the link that gets sent to your subscriber, if your form is set to double opt-in. You can find the IP info for each contact on the member details page:
Of course, these details are also included in your subscribers export files.
Activate the Signup IP addon by clicking on the Addons menu at the top of the page. Then click on Add more and scroll until you see that option. Turn it on and you're all set!