Transport: I??™ll use UDP. Send me the data.
Application: Here is the data.
Exploring the Packet Delivery Process 297
it must send the frame to its default gateway, where the frame can be forwarded. If the host
does not have a Layer 2 mapping for the default gateway, the host uses the standard ARP
process to obtain the mapping for the router. Figure 4-33 shows this process.
Figure 4-32 Transport Sends Data to IP Stack
Figure 4-33 Determine Host Is on a Different Subnet
The user has programmed the IP address of 192.168.3.2 as the default gateway. Host
192.168.3.1 sends out the ARP request, and it is received by the router. Figure 4-34 shows
the PC sending out an ARP request.
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
UDP: I??™ll put in a UDP header.
UDP: IP send this to 192.168.4.2.
IP: I??™ll put in a IP header.
IP: Layer 2 send this to 192.168.4.2.
SRC IP
192.168.3.1
DST IP
192.168.4.2
UDP
HDR
APP
DATA
APP
DATA
UDP
HDR
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
IP: I am on 192.
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