58 RabbitCore RCM3365/RCM33756.2.2 How IP Addresses are UsedThe actual hardware connection via an Ethernet uses Ethernet adapter addresses (alsocalled MAC addresses). These are 48-bit addresses and are unique for every Ethernetadapter manufactured. In order to send a packet to another computer, given the IP addressof the other computer, it is first determined if the packet needs to be sent directly to theother computer or to the gateway. In either case, there is an Ethernet address on the localnetwork to which the packet must be sent. A table is maintained to allow the protocoldriver to determine the MAC address corresponding to a particular IP address. If the tableis empty, the MAC address is determined by sending an Ethernet broadcast packet to alldevices on the local network asking the device with the desired IP address to answer withits MAC address. In this way, the table entry can be filled in. If no device answers, thenthe device is nonexistent or inoperative, and the packet cannot be sent.Some IP address ranges are reserved for use on internal networks, and can be allocatedfreely as long as no two internal hosts have the same IP address. These internal IPaddresses are not routed to the Internet, and any internal hosts using one of these reservedIP addresses cannot communicate on the external Internet without being connected to ahost that has a valid Internet IP address. The host would either translate the data, or itwould act as a proxy.Each RCM3365/RCM3375 RabbitCore module has its own unique MAC address, whichconsists of the prefix 0090C2 followed by a code that is unique to each RCM3365/RCM3375 module. For example, a MAC address might be 0090C2C002C0.TIP: You can always obtain the MAC address on your board by running the sample pro-gram DISPLAY_MAC.C from the SAMPLES\TCPIP folder.
