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GATE | GATE-CS-2015 (Set 1) | Question 65

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




Source address


Time to Live (TTL)



Answer: (B)


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

Two Scenarios for Source Address field:

  1. Device having a Public IP Address then IP header Source address field cannot be modified by any router. 
  2. Device having a Private IP Address then IP header Source address field can be modified only by NAT(Network Address Translation) capable router with the public IP address(Any typical router will not be able to process private ip address and thus drop the packet). Translation of Private IP to Public IP is done to form a communication between Sender & Receiver outside of their own network(LAN) or over Internet.

Only Source Address is what IP address can not change SO B is the answer.

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Last Updated : 28 Jun, 2021
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