Learn how a router selects the most specific route for forwarding packets based on the destination IP address and subnet mask. Understand the longest match principle in IP routing.
Table of Contents
Question
Refer to the exhibit.
How will the device handle a packet destined to IP address 100.100.100.100?
A. It will always prefer the static route over dynamic routes and choose the route
S 100.100.0.0/16 [1/0] via 192.168.4.1.
B. It will choose the route with the lowest metric,
R 100.0.0.0/8 [120/2] via 192.168.3.1, 00:00:13, Ethernet0/3.
C. It will choose the route with the highest metric,
D 100.100.100.0/24 [90/435200] via 192.168.2.1, 00:00:13, Ethernet0/2.
D. It will choose the route with the longest match,
O 100.100.100.100/32 [110/21] via 192.168.1.1, 00:05:57, Ethernet0/1.
Answer
Based on the provided routing table information, the device will choose the route with the longest match to handle a packet destined to IP address 100.100.100.100. The correct answer is:
D. It will choose the route with the longest match,
O 100.100.100.100/32 [110/21] via 192.168.1.1, 00:05:57, Ethernet0/1.
Explanation
The routing table contains various routes with different subnet masks. When a packet arrives, the router compares the destination IP address against each route in the routing table to find the best match. The route with the longest subnet mask (most specific) that matches the destination IP will be used.
In this case, there are four relevant routes:
S 100.100.0.0/16
R 100.0.0.0/8
D 100.100.100.0/24
O 100.100.100.100/32
The route O 100.100.100.100/32 has the longest subnet mask (/32) that exactly matches the destination IP 100.100.100.100. Therefore, this route will be chosen to forward the packet via next-hop 192.168.1.1 out interface Ethernet0/1.
The other routes have shorter subnet masks and are less specific, so they will not be preferred for this specific destination IP address.
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