Depends on how your ISP connection works. If they use DHCP, there is an IP address "checked out" to you that remains yours until some period of time of inactivity. If you come back within that time period, you get your old IP address back.
In other cases you might be connected to a local server (or one of several). If there is only one, you will get the same IP every time. Or it may vary, say among 4 choices.
or it may do something else entierely, approximating a random number generator.
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!