Determine which computers are causing the problems

Scenario/discussion:
What tool should you use to accomplish all the following based on scenario outlined below:
Determine which computers are causing the problems.
Record and examine network packets that are coming to/from specific machines.
View data related only to specific types of network packets.
As a consultant you must research a network-related problem at a small organization. The environment supports many custom-developed applications that are not well documented. A manager suspects that some computers on the network are generating excessive traffic and slowing down the network.
Comments from Customer
Discipline: Network Analysis