Question
Which one of the following fields of an IP header is NOT modified by a typical IP router?
Official answer and solution
- Official answer
- B
- From
- Official GATE 2015 answer key
Solution
Review the modifications performed by an intermediate IP router when forwarding an IPv4 packet.
When an intermediate router receives and forwards a standard IPv4 datagram:
- Time to Live (TTL): The router decrements the TTL field by at least 1 (Option C is modified).
- Header Checksum: Because the TTL field changes, the router must recompute and update the header checksum (Option A is modified).
- Total Length: If the packet size exceeds the next link's MTU and fragmentation is allowed, the router fragments the packet, changing the Total Length field in the fragment headers (Option D can be modified).
- Source IP Address: An intermediate router preserves the original sender's IP address. It never modifies the source IP address during standard routing (Option B is NOT modified).
Therefore, the Source address is not modified by a typical IP router (option B).
Final answer
B
Common trap
Thinking of Network Address Translation (NAT) gateways, whereas the question specifies a standard IP router.
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2015 Computer Science and Information Technology (CS) Session 1
- Question
- 32 of 65, printed as CS Q.22
- Section
- Computer Science and Information Technology
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