FWIW, and based on my sometimes not-so-great-memory:

When you log on your ISP assigns you a number. They keep a record of who, when, etc., for info to catch perps and pervs. The numbers used to be random, so when you logged off your number went back into the "pool."

If you have DSL and an ISP but never turn off your computer, your number stays the same. If you shut down and then power up again, you get a new number.

If you have a direct line (T100?) your number never changes, ever.

There are a few places on the net where you can verify your ID number. I think the sites offering Firewalls offer to check your system and in the process they show you your number as part of their pitch. If you want to check into your number(s), you might visit a couple of these at different times for verification.

Hope this helped.