Header Fields of an IP Packet

An IP (Internet Protocol) packet consists of a header followed by the data payload. The header contains various fields that provide information about the packet and help in routing it across the network. These fields are typically modified by IP routers as the packet is forwarded from the source to the destination.

Fields Modified by a Typical IP Router

1. Checksum: The IP header includes a checksum field which is used to detect errors in the packet during transmission. The checksum is calculated by the sender and verified by each router along the path. If errors are detected, the packet is discarded. Routers may update the checksum field if modifications are made to the IP header.

2. Source Address: The source address field in the IP header specifies the IP address of the sender. As the packet is forwarded through different routers, the source address remains unchanged. It is an essential field as it helps in identifying the origin of the packet.

3. Time to Live (TTL): The TTL field in the IP header indicates the maximum number of hops or routers the packet can traverse before being discarded. Each router that processes the packet decrements the TTL value by 1. If the TTL reaches 0, the packet is dropped. Routers must update the TTL field as they forward the packet to ensure its proper handling.

Field NOT Modified by a Typical IP Router

- Length: The length field in the IP header specifies the total length of the IP packet, including both the header and the data payload. This field is set by the sender and remains unchanged throughout the routing process. Routers do not modify the length field as they forward the packet.

Conclusion

In summary, among the given options, the field that is NOT modified by a typical IP router is the source address. The checksum, TTL, and length fields may be updated or modified by routers as they process and forward the IP packet.