GATE CS 2015 Set 1 Q22 — Fragmentation and IP Addressing, IPv4, CIDR

When an IP packet is processed and forwarded by a typical IP router, several fields in the IP header are modified:

• Time to Live (TTL): Decremented by at least $1$ at each router hop to prevent infinite routing loops.
• Header Checksum: Since the TTL field changes, the checksum must be recomputed and updated at each router.
• Length: If the packet size exceeds the Maximum Transmission Unit (MTU) of the outgoing link, the router fragments the packet, which modifies the Total Length field of the fragmented packets.

The Source Address specifies the original sender of the packet and remains unchanged as it traverses typical IP routers (excluding NAT devices, which perform address translation and are not considered standard typical routers in this context).

Therefore, the field that is NOT modified by a typical IP router is the Source address.