9.3 | Obtaining an IP address | |||
9.3.6 | Problems in address resolution |
One of the major problems in networking is
how to communicate with other network devices.
In TCP/IP
communications, a datagram on a local-area network must contain both a
destination MAC address and a destination IP address. These addresses
must be correct and match the destination MAC and IP addresses of the
host device. If it does not match, the datagram will be discarded by
the destination host. Communications within a LAN segment require two
addresses. There needs to be a way to automatically map IP to MAC
addresses. It would be too time consuming for the user to create
the maps manually. The TCP/IP suite has a protocol, called Address
Resolution Protocol (ARP), which can automatically obtain MAC
addresses for local transmission. Different issues are raised when data is sent outside
of the local area network.
Communications between two LAN segments have an additional task. Both the IP and MAC addresses are needed for both the destination host and the intermediate routing device. TCP/IP has a variation on ARP called Proxy ARP that will provide the MAC address of an intermediate device for transmission outside the LAN to another network segment.
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