Question

Imagine a routing table has the following (abbreviated) entries:
10.0.0.0 255.0.0.0 via s0/0/0
10.1.0.0 255.255.0.0 via s0/0/1
10.1.1.0 255.255.0.0 via s0/0/2
On which interface will a packet with a destination IP of 10.1.45.21 exit? How many entries above are a match for that address? Why would the router pick the one you chose (explain its logic).  

Answer 1

Destination : 10.1.45.21

On which interface will a packet with a destination IP of 10.1.45.21 exit?
Answer: Packet will exit at via s0/0/1


Explanation:

1) Starting with 10.0.0.0 255.0.0.0 via s0/0/0
Applying Subnet Mask 255.0.0.0 on 10.1.45.21 we get 10.0.0.0 --> It Matches here with 10.0.0.0

2) Starting with 10.1.0.0 255.255.0.0 via s0/0/1
Applying Subnet Mask 255.255.0.0 on 10.1.45.21 we get 10.1.0.0 --> It Matches here with 10.1.0.0​​​​​​​


3) Starting with 10.1.1.0 255.255.0.0 via s0/0/2
Applying Subnet Mask 255.255.0.0 on 10.1.45.21 we get 10.1.0.0 ---> Does not MATCH WITH 10.1.1.0

2)

How many entries above are a match for that address?
Answer: 2 Entries are Matched for the address

3) Why would the router pick the one you chose (explain its logic).  
Answer: Router will route it via s0/0/2 because 10.1.0.0 is the Longest prefix match because 10.0.0.0 matches only 10
where as 10.1.0.1 matches 10.1 which is the Longest Prefix match. Hence Router will route it via s0/0/2

PLEASE COMMENT if there is any concern.

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