Hi, Not sure if this is correct but my answer would have been something like this: Reason 1: The network layer primarily deals with packets so the sequencing numbers can be used to sequence the packets whereas in the data link layer, the packets are broken up into frames which are then seperately numbered. Reason 2: The sequence number also helps with flooding protocols. It prevents the same packet from being sent out again by the same router. When the router detects a sequence number that has appeared before, it just discards the packet. Thanks, would love some feedback too!