|If an ISP arranges a payment system with his customers, or if a company wants to divide the costs between separate departments, you will always have to use one of the following mechanisms:|
Port based billing:
If you can assign one port of the distributing device of your network to each "customer", this will be the simplest way to bill them. All traffic (bytes) transported to or from the appropriate port can simply be read from the device via SNMP and is to be multiplied by the price per byte.
In this case you can't differentiate the costs by the service used (IP-Port), or the actual partner (IP- resp. MAC-address). But if the customer is connected via a manageable switch resp. router, the counters for In und Out (BytesIn and BytesOut) of the connecting device can be taken as measured without extra costs for special hardware.