Can you please tell us more about your requirement, Do you want to see the actual traffic / connections pertaining to the users or do you just want to know how the IPs have been allocated. For the first option netflow is the recommend way to go, you may also look at www.fireplotter.com (real-time).There are a number of free netflow analyzer available on the Internet.
For the second option, syslogs are sufficient. Any good syslog analyzer (or even SIEM) would give you this information or would let you write a brief parser/rule if its not already covered (e.g. MARS, Loglogic etc).
Regards
Farrukh
