Computers have a unique hardware address that is allocated to the network interface when the machine is manufactured. When a computer is connected to a LAN, the Address Resolution Protocol provides a mapping of IP addresses to the hardware or MAC address. This allows each computer on that network to recall the address of every other machine connected to the same LAN. The benefit of this system is that it reduces traffic on the LAN, because there is no need to query the address of another machine every time correspondence is exchanged. A computer will usually retain the same MAC address if it is moved to another network, although it may change if the hardware is upgraded or altered.