168.4.2. The application selects a User Datagram Protocol (UDP) as the transport for
this data, indicating that the reliability of delivery is unimportant for this application or
handled by application.
Figure 4-31 Application Sends Data to Transport Stack
Because it is not necessary to set up a session, the application can start sending data. UDP
prepends a UDP header and passes the protocol data unit (PDU) to IP (Layer 3) with an
instruction to send the PDU to 192.168.4.2. IP encapsulates the PDU in a Layer 3 packet
and passes it to Layer 2. This process is illustrated in Figure 4-32.
This example differs from previous examples of sending packets (now called frames) at
Layer 2 because the two hosts are on different segments: 192.168.3.0/24 and 192.168.4.0/24.
Because the host is con?¬?gured with an IP address and a subnet mask, it understands that
192.168.4.0 is on a different network. Because it does not know how to get to this network,
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.3.2
L2 = 0800:0333:2222
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.4.2
L2 = 0800:0222:1111
APP
DATA
Application: Network, I have some data to send to
192.168.4.2, and I don??™t need a reliable connection.
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