I know these numbers are related to the MTU size but I did not get a clarity of what exactly it is and why this number.Could anyone please explain?

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1 Answer 1


68 bytes is the minimum size of IPv4 datagram every device must be able to forward without further fragmentation.

576 bytes is the minimum size of IPv4 datagram every device has to be able to receive (it can be whole or fragmented).

(according to RFC 791, page 24)

  • Consider two routers connected in series. First router(R1) receives fragments of 68 bytes. It just forwards them to R2. Since 576 bytes is the minimum size of IPv4 datagram it can receive, will it drop all the fragments? Jun 13, 2016 at 2:10
  • 1
    No, because 1) the router doesn't receive the fragments, it's not the destination host, it just forwards the fragments onto the next hop network. 2) the receiving host doesn't receive a datagram, it receives fragments. When these fragments don't add up to more than 576 bytes in the resulting datagram the IPv4 host has to be able to handle.it. Jun 13, 2016 at 12:51

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