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Three models exist for communication between hosts on a
network whether or not the network is an intranet or the Internet.
The first model is the unicast model, which is one-to-one
communication. In Figures 1-4a through 1-4c, one host desires to
send traffic to another specific host on the same IP subnet (IP
addressing and subnets are covered in detail in Chapter 2, “Internet Protocol (IP),
Unicast, Broadcast and Multicast Addresses”). For the ethernet Local Area Network
(LAN), the hosts
must contend with two different address schemes. The first scheme is
the ethernet address that is burned into the Network Interface Card
(NIC). The ethernet
address is a six-byte (48-bit) link layer address that is globally
unique and cannot be changed. Because the ethernet address is burned
into the NIC, the ethernet address of the host changes if the NIC is
changed. We have seen that on an ethernet LAN all data traffic is
encapsulated in frames. Even though the host is sending to an IP
address, the IP packet must be encapsulated in an ethernet frame. To
accomplish the encapsulation, the sending host must resolve the
receiving host’s IP to ethernet address mapping. The mapping is
accomplished using the Address Resolution Protocol (ARP). |
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