Role of the data link layer

The data link layer moves a frame across one link: framing, MAC addressing (48-bit flat card names, 6 bytes), error detection, and access control on shared links. A packet crossing h links travels inside h different frames; only the MAC addresses change, never the network-layer addresses inside.

1. NAT · 1 mark

   A packet crosses 4 routers between two hosts. How many separate frames carry it?

   Write your answer before opening the key.

2. MCQ · 1 mark

   Which address is flat (no location information)?

   1. MAC address
   2. Network-layer address
   3. Both
   4. Neither
3. NAT · 1 mark

   How many MAC addresses can one 24-bit vendor prefix cover?

   Write your answer before opening the key.

4. MCQ · 1 mark

   Which layer chooses the route, and which carries the unit across one hop?

   1. Data link; network
   2. Network; data link
   3. Transport; network
   4. Physical; data link
5. MCQ · 2 marks

   Bit stuffing for data 0111110 (flag 01111110) produces:

   1. 01111100
   2. 0111110
   3. 011111010
   4. 00111110
6. MSQ · 1 mark

   Which change at every router hop?

   1. Source MAC address
   2. Destination MAC address
   3. Destination network-layer address
   4. The frame itself
7. NAT · 1 mark

   Bytes in a MAC address?

   Write your answer before opening the key.

8. MCQ · 1 mark

   Store-and-forward at a router means it receives the whole ___ before forwarding.

   1. frame
   2. bit
   3. message
   4. segment