How to protect registered IP addresses

Hi, Internet Experts,

I run into a problem these days. We have a campus network with a class B
IP block. This network is connected with Internet. By now, we have
thousands of registered users and allocate IP addresses to them.
Recently, we installed an accouting system to monitor how much traffic
each user had accounted for. Each month, these users should pay their
bills based on how much traffic they have used.

Soon, we find some problems: Some guys are using the unallocated
addresses, and they are accessing Internet wildly; At the same time,
other anonymous users are using illegally the addresses of registered

Here my questions are:

First, Is there any better solution to protect the unallocated addresses
--- besides access-list? The first selection seems to be adding
access-lists on the routers, to block all the unallocted addresses.
However, considering the quantity of the IP addresses(a class B), it
sure is a great burden to block the addresses one by one(or almost one
by one). I'm not sure if cisco routers support such long access-list.

Second, how can we protect the IP addresses of registered users from
being used by other people ?

Any tips are greatly appreciated!


Wei Sun