Sorry, you selected the wrong answer.	Why must the users of a network have addresses?
	needed as a port assignment so they can make and receive calls required for billing

Some networks may use a port assignment to create an address, but that is not the main reason for addresses.

Some networks may require that all calls have an address so they can bill someone, but that is not the main reason for addresses.

The correct answer is "So they can make and receive calls." When each terminal on a packet network has a address, then each one can call the other by using their address. This is very much like the operation of the telephone network. Everyone has a telephone number - so anyone can make calls as long as they know the phone numbers.