Assume that source S and destination D are connected through two intermediate routers labelled R. Determine how many times does each packet has to visit the network layer and the data link layer during a transmission from S to D.

From above given diagram, it is clearly visible that packet will visit network layer 4 times, once at each node [S, R, R, D] and it will visit Data Link layer 6 times. One time at S and one time at D, then two times for each intermediate router R as data link layer is used for link-to-link communication. Once a packet reaches R and goes up from physical-DL-Network and second time when packet comes out of router in the order Network-DL-Physical.