4. Multi-user systems
The trickiest to handle are multi-user systems, for instance general-purpose servers such as the OUCS GNU/Linux service. Such systems permit concurrent usage by multiple users. Utilities such as process accounting allow for some degree of per-user accountability.
Some servers are more specific in purpose, for instance mail relays or web proxies. These may authenticate users, and if not should at the very least log the IP address from which a connection originated.