How does a router determine the most specific route when it receives multiple valid routes in its routing table?
When a router receives multiple valid routes in its routing table, it follows a process to determine the most specific route for forwarding packets. This process is important in ensuring efficient and accurate routing in computer networks.
Routers use a concept known as the longest prefix match to determine the most specific route. The longest prefix match involves comparing the destination IP address of the incoming packet with the entries in the routing table. The router selects the route with the longest matching prefix as the most specific route to forward the packet.
Each entry in the routing table consists of a destination network address and a subnet mask. The subnet mask determines the number of bits in the network portion of the IP address. When a packet arrives at the router, the router performs a bitwise logical AND operation between the destination IP address of the packet and the subnet mask of each entry in the routing table.
The router then compares the result of this operation with the destination network address in each routing table entry. The router selects the entry with the longest matching prefix as the most specific route. In other words, the router selects the route that covers the largest range of IP addresses that includes the destination IP address of the packet.
For example, consider a router with the following entries in its routing table:
– Route 1: Destination network = 192.168.1.0/24
– Route 2: Destination network = 192.168.1.128/25
– Route 3: Destination network = 192.168.1.160/27
If a packet with the destination IP address 192.168.1.175 arrives at the router, the router will perform the longest prefix match as follows:
– Route 1: 192.168.1.0 & 255.255.255.0 = 192.168.1.0
– Route 2: 192.168.1.0 & 255.255.255.128 = 192.168.1.0
– Route 3: 192.168.1.0 & 255.255.255.160 = 192.168.1.0
In this case, Route 3 has the longest matching prefix (192.168.1.160/27) that includes the destination IP address 192.168.1.175. Therefore, the router will select Route 3 as the most specific route to forward the packet.
Routers determine the most specific route by using the longest prefix match algorithm, which involves comparing the destination IP address of incoming packets with the entries in the routing table and selecting the route with the longest matching prefix.
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