I was reading pages 18-21 in RFC 791 about the Internet Protocol and in the sections on Loose Source and Record Route (LSRR), and Strict Source and Record Route (SSRR), and Record Route (RR) it says that the smallest value for the pointer is 4. To confirm my understanding of how this pointer works - is it really 1-indexed?

  • Based on the information at masterraghu.com/subjects/np/introduction/… it certainly seems it is. I haven't verified this though. Mar 7, 2014 at 21:59
  • Thank you for the pointer to documentation! Looks interesting and informative. I like that they illustrated aligning the addresses on 32-bit boundaries with a NOP option. Mar 7, 2014 at 22:03
  • -- Hmm, seems it may be a quote from Unix Network Programming, by W. Richard Stevens. Well, guess I'd better dust off my copy and read it. Mar 7, 2014 at 22:14
  • Sander, if you write up your comment as an answer, I will accept it. Your reference answered this question, and probably my next hundred questions. Mar 7, 2014 at 22:30

1 Answer 1


Converting my comment to an answer, as requested :)

Based on the information in UNIX® Network Programming Volume 1, Third Edition: The Sockets Networking API, section 27.3 it certainly seems it is. I haven't verified this though.

Example diagram of a source routing header

Based on this diagram and this part of the description: "ptr is a pointer which contains the offset of the next IP address to be processed in the route, and we initialize it to 4, which points to the first IP address.", the ptr field does indeed seem to be 1-indexed. Note that the NOP option isn't counted. The option, and therefore the counting, starts from code.


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