A machine receives an IPv4 datagram. The protocol field of the IPv4 header has the protocol number of a protocol X. Which ONE of the following is NOT a possible candidate for X? A) Internet Control Message Protocol (ICMP) B) Internet Group Management Protocol (IGMP) C) Open Shortest Path First (OSPF) D) Routing Information Protocol (RIP)

Official answer: D) Routing Information Protocol (RIP). Explanation: The 8-bit Protocol field in the IPv4 header indicates the higher-layer or next-level protocol whose payload is encapsulated directly within the IPv4 datagram. - ICMP (Internet Control Message Protocol): Encapsulated directly in IPv4 datagrams with IP protocol number 1. - IGMP (Internet Group Management Protocol): Encapsulated directly in IPv4 datagrams with IP protocol number 2. - OSPF (Open Shortest Path First): Encapsulated directly in IPv4 datagrams with IP protocol number 89. - RIP (Routing Information Protocol): Operates on top of the UDP transport layer protocol (using UDP port 520). Therefore, the Protocol field of an IPv4 datagram carrying RIP packets contains the protocol number for UDP (17), not RIP. Thus, RIP cannot be directly specified in the IPv4 header's Protocol field, making D the correct option.

A) Internet Control Message Protocol (ICMP)

B) Internet Group Management Protocol (IGMP)

C) Open Shortest Path First (OSPF)

IPv4, CIDR and NAT

GATE CSE 2025 Set 2 · Q18

MCQ1 markEasy

Time00:00

A machine receives an IPv4 datagram. The protocol field of the IPv4 header has the protocol number of a protocol X. Which ONE of the following is NOT a possible candidate for X?

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Solution

Step-by-step verified solution

Official answer: D) Routing Information Protocol (RIP)

IPv4, CIDR and NAT

The 8-bit Protocol field in the IPv4 header indicates the higher-layer or next-level protocol whose payload is encapsulated directly within the IPv4 datagram.

ICMP (Internet Control Message Protocol): Encapsulated directly in IPv4 datagrams with IP protocol number $1$ .
IGMP (Internet Group Management Protocol): Encapsulated directly in IPv4 datagrams with IP protocol number $2$ .
OSPF (Open Shortest Path First): Encapsulated directly in IPv4 datagrams with IP protocol number $89$ .
RIP (Routing Information Protocol): Operates on top of the UDP transport layer protocol (using UDP port 520). Therefore, the Protocol field of an IPv4 datagram carrying RIP packets contains the protocol number for UDP ( $17$ ), not RIP.

Thus, RIP cannot be directly specified in the IPv4 header's Protocol field, making D the correct option.

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This IPv4, CIDR and NAT previous year question is Q18 of the GATE CSE 2025 Set 2 paper. Find more GATE CSE PYQs on IPv4, CIDR and NAT in Computer Networks, or browse all GATE CSE PYQs.

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IPv4, CIDR and NAT in GATE CSE

IPv4, CIDR and NAT has 14 GATE CSE PYQs across 10 papers from 2012 to 2025. 5 of them are 1-mark MCQs like this one, and it is the only one in GATE CSE 2025 Set 2.

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.