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+25 votes

Which of the following fields of an IP header is NOT modified by a typical IP router?

  1. Check sum
  2. Source address
  3. Time to Live (TTL)
  4. Length
in Computer Networks by Boss (30.8k points) | 2.9k views
@Kapilp how is length modified
If you fragment a datagram into smaller fragments then offcourse size of IP datagram becomes smaller and so Total_Length field of IP datagram will be modified...
doubt here....Because TTL value modified by Hop Only, not by router so i think both B and C should be right....

@vikas Hops are links they are not the electronic devices which can change the value.

6 Answers

+31 votes
Best answer
Source Address.
by Veteran (431k points)
edited by
Is checksum modified by IP router?
Sir won't you consider the NAT router. In that case the NAT router changes the source address to its own ip address and sends the packet out of the orgorganisation..
Question says "typical" :)
Sorry I missed the word!
How is length modified?
@habibkhan how is length modified
during fragmentation
@Arjun Sir,

How checksum is modified by router?

It may be modified by error in Transmission line but how by router?

Sorry,when it breaks into fragments each fragment add original header plus extra data,by this checksum got modified.
when time to live changes.

it's obvious there will be modification in checksum :)
+4 votes
B. Source Address
by Active (1.8k points)
+2 votes
Only source IP address will not change at router and all mentions feild above will definately change.

Therefore option B will be right option for it.
by Boss (10.2k points)
edited by
+1 vote
Since it has been asked for IP Header so Source adress will not change but if it was about the frame than source address (i.e MAC Address) is changed.
by (93 points)
0 votes

Ans B

Length and checksum can be modified when IP fragmentation happens. Time to live is reduced by every router on the route to destination.

by Junior (621 points)
–1 vote
Only source Ip address will not change otherwise our delivered letter can not go our girl freind house.
by Boss (10.2k points)
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