Which one of the following fields of an IP header is NOT modified by a typical…

Answer: The source address is not modified by a typical IP router.

Why:

• Time to Live (TTL): Routers decrement the TTL by 1 at each hop to prevent routing loops; this field is therefore modified by routers.

• Header Checksum (IPv4): Because routers change the TTL (and possibly other header fields), the IPv4 header checksum must be recalculated by the router. Note that IPv6 does not include a header checksum.

• Length: Normally remains the same during forwarding. However, if a router must fragment a packet to fit the outgoing MTU, the lengths of resulting fragments will differ, so length can be changed in that special case.

• Source address: A standard router forwards packets without changing the source IP address. Only special functions such as Network Address Translation (NAT) or application-level proxies modify the source address.

Conclusion: Because routers routinely modify the TTL and (in IPv4) the header checksum, and may change length during fragmentation, the source address is the field that is not modified by a typical IP router.