How Ip Addresses Are Used - Rabbit RCM3000 User Manual

6.4 How IP Addresses are Used

The actual hardware connection via an Ethernet uses Ethernet adapter addresses (also
called MAC addresses). These are 48-bit addresses and are unique for every Ethernet
adapter manufactured. In order to send a packet to another computer, given the IP address
of the other computer, it is first determined if the packet needs to be sent directly to the
other computer or to the gateway. In either case, there is an IP address on the local net-
work to which the packet must be sent. A table is maintained to allow the protocol driver
to determine the MAC address corresponding to a particular IP address. If the table is
empty, the MAC address is determined by sending an Ethernet broadcast packet to all
devices on the local network asking the device with the desired IP address to answer with
its MAC address. In this way, the table entry can be filled in. If no device answers, then
the device is nonexistent or inoperative, and the packet cannot be sent.
IP addresses are arbitrary and can be allocated as desired provided that they don't conflict
with other IP addresses. However, if they are to be used with the Internet, then they must
be numbers that are assigned to your connection by proper authorities, generally by dele-
gation via your service provider.
Each RCM3000 RabbitCore module has its own unique MAC address, which consists of
the prefix 0090C2 followed by the code that appears on the label affixed to the RCM3000
module. For example, a MAC address might be 0090C2C002C0.
TIP: You can always verify the MAC address on your board by running the sample pro-
gram DISPLAY_MAC.C from the SAMPLES\TCPIP folder.
40 RabbitCore RCM3000
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