Rabbit RabbitCore RCM4000, User Manual

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68 RabbitCore RCM4000
6.2.2 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 Ethernet address on the local
network 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.
Some IP address ranges are reserved for use on internal networks, and can be allocated
freely as long as no two internal hosts have the same IP address. These internal IP
addresses are not routed to the Internet, and any internal hosts using one of these reserved
IP addresses cannot communicate on the external Internet without being connected to a
host that has a valid Internet IP address. The host would either translate the data, or it
would act as a proxy.
Each RCM4000 RabbitCore module has its own unique MAC address, which consists of
the prefix 0090C2 followed by a code that is unique to each RCM4000 module. For exam-
ple, a MAC address might be 0090C2C002C0.
TIP: You can always obtain the MAC address on your module by running the sample
program
DISPLAY_MAC.C
from the
SAMPLES\TCPIP
folder.