Which device validates the IP checksum and recalculates before forwarding?

• A. End hosts
• B. Routers
• C. Switches
• D. DNS servers

Answer: (B)

IP header integrity is maintained as a packet moves through a network, and the device that both checks the header and adjusts it for forwarding is the router. When a router receives a packet, it verifies the IPv4 header checksum to ensure the header hasn’t been corrupted. It then forwards the packet toward its destination, and if it decrements the Time-To-Live (TTL) field during this hop, it must update the header accordingly and recalculate the checksum so the next device sees a valid header. End hosts primarily process the packet they receive and don’t forward it, so they don’t recalibrate the header for forwarding. Switches at Layer 2 focus on MAC-based forwarding and typically don’t modify or recalculate IP header checksums unless they’re performing Layer 3 routing. DNS servers operate at the application layer and don’t participate in IP header checksum processing for forwarding.