GATE2025_CS1_Q40

A packet with the destination IP address 145.36.109.70 arrives at a router whose routing table is shown. Which interface will the packet be forwarded to?

Subnet Address	Subnet Mask (CIDR)	Interface
145.36.0.0	/16	E1
145.36.128.0	/17	E2
145.36.64.0	/18	E3
145.36.255.0	/24	E4
Default	--	E5

✅ Final Answer:

The correct answer is option (A) E3.

🔹 Short Answer:

Routers use the Longest Prefix Match rule. We check the destination IP 145.36.109.70 against each entry. It matches both the /16 network (to E1) and the /18 network (to E3). Since /18 is a longer (more specific) prefix than /16, the router chooses the route to interface E3.

🔸 Long Answer:

To determine the correct outgoing interface, a router performs a bitwise AND operation between the destination IP address and the subnet mask of each entry in its routing table. It then compares the result with the subnet address of that entry. If multiple routes match, the router uses the **Longest Prefix Match** rule.

Step 1: Convert the Destination IP to Binary

The destination IP is 145.36.109.70. Since our masks are /16, /17, /18, and /24, the second and third octets are the most important for comparison.

145	36	109	70
10010001	00100100	01101101	01000110

Step 2: Check Each Route in the Table

Route 1: 145.36.0.0 /16 to E1

• The /16 mask means we check the first 16 bits.
• Destination IP's first 16 bits: 10010001.00100100 (145.36)
• Subnet Address's first 16 bits: 10010001.00100100 (145.36)
• Result: MATCH (Prefix length: 16)

Route 2: 145.36.128.0 /17 to E2

• The /17 mask means we check the first 17 bits.
• Destination IP's first 17 bits: 10010001.00100100.0... (from 01101101)
• Subnet Address's first 17 bits: 10010001.00100100.1... (from 10000000)
• The 17th bits (marked in bold) are different.
• Result: NO MATCH

Route 3: 145.36.64.0 /18 to E3

• The /18 mask means we check the first 18 bits.
• Destination IP's first 18 bits: 10010001.00100100.01... (from 01101101)
• Subnet Address's first 18 bits: 10010001.00100100.01... (from 01000000)
• The first 18 bits are identical.
• Result: MATCH (Prefix length: 18)

Route 4: 145.36.255.0 /24 to E4

• The /24 mask means we check the first 24 bits (3 octets).
• Destination IP's first 24 bits: 145.36.109
• Subnet Address's first 24 bits: 145.36.255
• The third octets (109 and 255) are different.
• Result: NO MATCH

Step 3: Apply the Longest Prefix Match Rule

We have found two matching routes:

1. To Interface E1 with a prefix length of 16.
2. To Interface E3 with a prefix length of 18.

The Longest Prefix Match rule states that the router must choose the most specific route, which is the one with the longest subnet mask (highest prefix number). Since 18 > 16, the router will forward the packet to interface E3.

Bibliography:
- Kurose, J. F., & Ross, K. W. (2016). Computer Networking: A Top-Down Approach. Pearson. (Chapter 4: The Network Layer).
- Tanenbaum, A. S., & Wetherall, D. J. (2010). Computer Networks. Prentice Hall. (Chapter 5: The Network Layer).

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