Is that true? I’m not sure it is. I’m pretty sure that if
microsoft.com registered for your service, you might find that their domain has multiple IP addresses, before you even get on to the difference between
support.microsoft.com and all the rest. Round-robin load balancing allows a DNS server to give out different IPs for the same domain, for example, or it did last time I was looking at stuff like this.
What if the connection comes from someone who has a dynamic IP address, and has a different one each time they connect? Or what if someone on a shared connection (shared router, for example) signs up for your service, but all the other people on that shared router do not?
IP addresses are rarely a good way to track users.