The simplest way would be to just run an analyzer on the web server logs. There are many (free) analyzers out there. Just do a search.

Bear in mind though that all you'll be getting is whatever IP address the last proxy in the request chain was using at the time. IP addresses do not identify machines. IP addresses are assigned to interfaces. One machine may have multiple interfaces (even the simplest setup will typically have at least two interfaces: 127.0.0.1 for the loopback interface is also an IP address). One interface may have multiple IP addresses. And IP addresses may move.