If an IPv4 packet exceeds the network MTU and the Don't Fragment (DF) bit is not set, what happens to the packet?

If an IPv4 packet exceeds the network MTU and the Don't Fragment (DF) bit is not set, what happens to the packet?

• A. It is dropped by the router.
• B. It is fragmented into smaller packets.
• C. It is encapsulated in a new VPN tunnel.
• D. It is queued until MTU can be increased.

Answer: (B)

When an IPv4 packet is larger than the MTU of the next link and the Don't Fragment bit is not set, routers break the packet into smaller pieces that fit the MTU. Each fragment carries part of the payload and includes fragmentation metadata (offset, total length, MF flag) so the destination can reassemble them in the correct order. Reassembly happens at the final destination, assuming all fragments arrive; if a fragment is lost, the entire original packet can fail to reassemble. If the Don't Fragment bit were set, the router would drop the packet and send back an ICMP fragmentation-needed message instead. The other options—dropping without fragmentation, encapsulating in a VPN tunnel, or queuing until MTU changes—do not apply when fragmentation is allowed.

When an IPv4 packet is larger than the MTU of the next link and the Don't Fragment bit is not set, routers break the packet into smaller pieces that fit the MTU. Each fragment carries part of the payload and includes fragmentation metadata (offset, total length, MF flag) so the destination can reassemble them in the correct order. Reassembly happens at the final destination, assuming all fragments arrive; if a fragment is lost, the entire original packet can fail to reassemble.

If the Don't Fragment bit were set, the router would drop the packet and send back an ICMP fragmentation-needed message instead. The other options—dropping without fragmentation, encapsulating in a VPN tunnel, or queuing until MTU changes—do not apply when fragmentation is allowed.