The ARP table is created and maintained dynamically, adding and changing address
relationships as they are used on the local host. The entries in an ARP table usually
expire after a period of time, by default 300 seconds; however, when the local host
wants to transmit data again, the entry in the ARP table is regenerated through the ARP
process.
Host-to-Host Packet Delivery
In Figure 1-63, an application on the host with a Layer 3 address of 192.168.3.1 wants
to send some data to the host with a Layer 3 address of 192.168.3.2. The application
wants to use a reliable connection. The application requests this service from the
transport layer.
The transport layer selects TCP to set up the session. TCP initiates the session by passing
a TCP header with the SYN bit set and the destination Layer 3 address (192.168.3.2) to the
IP layer.
90 Chapter 1: Building a Simple Network
Figure 1-63 Packet Delivery
The IP layer encapsulates the TCP??™s SYN in a Layer 2 packet by prepending the local Layer
3 address and the Layer 3 address that IP received from TCP. IP then passes the packet to
Layer 2. Figure 1-64 shows this operation.
Figure 1-64 IP Layer Operation
Layer 2 needs to encapsulate the Layer 3 packet into a Layer 2 frame.
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