IPv4 Host Routing
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Hosts actually use some simple routing logic when choosing where to send a packet. If you
assume that the design uses subnets (which is typical), this two-step logic is as follows:
Step 1. If the destination IP address is in the same IP subnet as I am, send the packet
directly to that destination host.
Step 2. Otherwise, send the packet to my default gateway, also known as a default
router. (This router has an interface on the same subnet as the host.)
For example, consider Figure 4-10 and focus on the Ethernet LAN on the left. When PC1
sends an IP packet to PC11 (220.127.116.11), PC1 first considers some match related to subnetting.
PC1 concludes that PC11’s IP address is in the same subnet as PC1, so PC1 ignores its default
router (Core, 18.104.22.168), sending the packet directly to PC11, as shown in Step 1 of the figure.
Figure 4-10 Host Routing: Forwarding to a Host on the Same Subnet
Alternatively, when PC1 sends a packet to PC2 (22.214.171.124), PC1 does the same kind of
subnetting math, and realizes that PC2 is not on the same subnet as PC1. So, PC1 forwards
the packet (Step 2) to its default gateway, 126.96.36.199, which then routes the packet to PC2.