All devices and computers that connect to an Ethernet network have a unique MAC address. It is rare, and usually not desirable, for a MAC address of a device to change. An IP address is an assigned number to a specific device. It can be re-assigned to another device at any time. A port number is another specified parameter and is utilized to route Ethernet packets. Routers use port numbers in packets originating outside the LAN to direct them to individual computers within the LAN. Software uses port numbers to route, accept, or reject packets.

Routers and PCs automatically detect and store in a table, the MAC addresses and the corresponding IP addresses of devices connected to the LAN. Therefore, MAC addresses are not important to virtually all software applications; they require only the IP address of the device in order to communicate. When an application creates a packet with the IP address, the network card on the PC references an internal table, adds the MAC address to packet, and subsequently, sends it out on the Ethernet. The packet is presented to all the devices on the LAN, but only the device with the matching MAC address receives it.

PCs and routers use Address Resolution Protocol (ARP) to match IP addresses with MAC addresses. When a packet is seen on the Ethernet with an unknown MAC address, these devices immediately send out an ARP Request packet requesting the MAC address of the sender. The device with the matching IP address will reply with an ARP Response with this parameter. Most computers and routers on the LAN will monitor this exchange and retain the information in its ARP table.

Sometimes a PC will not allow software to send a packet to an IP address which has not been saved in its ARP table. This is sometimes the case when a new device is added to a network but it has not transmitted a packet, thus not evoking an ARP request. In this case, it may be necessary to manually enter the IP and MAC address into the ARP table. EL devices will only accept UDP packets containing its MAC address or UDP broadcast packets which contain a generic broadcast MAC address.