do ISPs keep track of end-user IP changes within thier network?

[Carlos Kamtha]

Hi, 

just a general curiousity question. it's been a long time since ive worked at 
an ISP. 

back then it was non-expiring DHCP leases and in some cases static IP for all.. 
(yes it was long ago..) 

Any feedback would be greatly appreciated..

Carlos.