To do this, Layer 2
needs to map the Layer 3 destination address of the packet to its MAC address. It does this
by requesting a mapping from the ARP program.
ARP checks its table. In this example, it is assumed that this host has not communicated
with the other host, so you see no entry in the ARP table. This results in Layer 2 holding
the packet until ARP can provide a mapping. Figure 1-65 shows this operation.
Layer 3 = 192.168.3.1
Layer 2 = 0800:0222:2222
Layer 3 = 192.168.3.2
Layer 2 = 0800:0222:1111
Application: Network, can you set up reliable
connection to 192.168.3.2. for me?
Transport: I??™ll use TCP.
Transport: TCP, set up a
session to 192.168.3.2.
TCP: IP, send this TCP
SYN to 192.168.3.2.
TCP
SYN
IP: Layer 2, send this packet to 192.168.3.2.
SRC IP
192.168.3.1
DST IP
192.168.3.2
TCP
SYN
Layer 3 = 192.168.3.1
Layer 2 = 0800:0222:2222
Layer 3 = 192.168.3.2
Layer 2 = 0800:0222:1111
Exploring the Packet Delivery Process 91
Figure 1-65 ARP Table Lookup
The ARP program builds an ARP request and passes it to Layer 2, telling Layer 2 to send
the request to a broadcast (all Fs) address. Layer 2 encapsulates the ARP request in a Layer
2 frame using the broadcast address provided by ARP as the destination MAC address and
the local MAC address as the source.
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