IST 220 – Sample Test 3 - Answer
Name: ______
- For each question (3 points), there are multiple choices, but only one is correct
1.1) The IP address 191.192.193.194 belongs to class _____
(a) A (b) B (c) C
Answer: [b]
1.2) The job of a router is to store and forward packets. In order to store packets, a router uses a ____
(a) packet table (b) routing table (c) forwarding table (d) queue (e) NIC with more memory
Answer: [d]
1.3) A router is different from a LAN switch because _____
(a)The router uses a forwarding table, but the LAN switch does not
(b)The router uses hierarchical addresses, but the LAN switch does not
(c)The router uses physical addresses, but the LAN switch does not
(d)The router forwards packets, but the LAN switch does not
Answer: [b]
1.4) In OSI seven-layer model, the routing layer is called ______
(a) data link layer
(b) physical layer
(c) transport layer
(d) network layer
(e) application layer
Answer: [d]
- Please say TRUE or FALSE to each of the following statements ( 3 points each)
2.1) __NA
2.2) ___True____ A computer cannot be assigned with the IP address 222.200.0.0 since the IP address is a network address.
2.3) ___false____ Since a phone switch is a special switch, it also does routing using a routing table.
2.4)__false_____ In OSI seven-layer-model, NIC layer is called network interface layer.
2.5)__true_____ An internetwork can be smaller than a WAN.
2.6)__true____ Since IP addresses are hierarchical addresses, IP addresses can make a routing table much smaller than using physical addresses.
2.7) ___true____ “Nest headers” imply that the IP payload of a packet is always a part of the NIC layer payload of the packet.
2.8) ______NA
2.9) ______NA
- Consider the following internetwork. Assume the Physical Address of NIC-A is E78D45C4FD66, of NIC-E is 7E8D4C544DBB. Assume the IP address of NIC-A is 121.22.230.2, of NIC-C is 192.5.2.2, of NIC-13 is 188.11.135.112, of NIC-E is 200.80.60.40. Assume the distance between R1 and R2 is 90 miles, between R1 and R3 is 150 meters, between R2 and R3 is 80 miles, between R2 and R4 is 300 miles, between R3 and R4 is 400 miles. Note that 1 mile is 1600 meters.
Assume computer E now wants to send the message “ER” to computer A. Assume the ASCII code for 'E' is 45, for 'R' is 52. Assume there are no errors during the transmission.
3.1) How many physical networks are there? Note that leased lines do not cause more networks. (3 points)
Answer: 4. Note that the segment where the bridge stays and the segment where computer E stays belong to the same physical network.
3.2) What is device W? (3 points)
Answer: W is a CSU/DSU
3.3) Please list all class C networks in this internetwork. (3 points)
Answer: [a] The physical network where computer C stays
[b] The physical network where E and the bridge both stay
3.4) What is the prefix of the IP address of NIC-42? (3 points)
Answer: 200.80.60
3.5) What is the network address of the network where computer D is attached? (3 points)
Answer: 188.11.0.0
3.6) To send out the message, computer E will first build the frame. Please draw the format of the frame when it leaves NIC-E. (3 points)
Answer:
| preamble| NIC42’s MAC | 7E8D4C544DBB | 121.22.230.2 | 200.80.60.40 | ER |CRC |
3.7) After a while, P-B will get the frame. What will P-B do? (3 points)
Answer: P-B will use the destination address field of the packet’s network interface layer header (i.e. NIC layer header) to determine whether to forward this packet or not. Since the destination is on another segment, P-B will forward the packet to NIC-B2.
3.8) After a while, P-4 will get the frame. Before P-4 does anything on the packet, what is the routing layer payload of this frame? (3 points)
Answer: | ER |
3.9) When this packet leaves P-4, what is the NIC layer header of this frame? ( 3 points)
Answer: | NIC23’s MAC | NIC41’s MAC|
3.10) What is the routing table inside R3? (4 points)
Answer:
Destination address / Where to go121.0.0.0 / NIC-31
188.11.0.0 / Local
192.5.2.0 / NIC-32
200.80.60.0 / NIC-32
3.11) After a while, P-1 will get the packet. When P-1 gets the packet, what is the format of the frame? (3 points)
Answer: | NIC13’s MAC | NIC31’s MAC | 121.22.230.2 | 200.80.60.40 | ER |
3.12) What will P-1 do next? (3 points)
Answer: [a] P-1 will strip off soh and eot.
[b] P-1 will strip off the network interface layer header.
[c] P-1 will do routing based on R1’s routing table. Based on the routing table, he should forward the packet to NIC-11 next.
[d] P-1 will add a new network interface layer header, which is: | NIC-A’s MAC | NIC11’s MAC |
[e] P-1 will add soh and eot
[f] P-1 will forward the packet to NIC-11
3.13) After a while, P-A will get the message. During the whole message passing process, how many processors have seen a routing layer header? Please consider all the computers and routers. (3 points)
Answer: 7 (note that P-E and P-B are included)
3.14) During the whole process, how many times is the router layer header changed? (3 points)
Answer: zero
3.15) During the whole process, how many times is the message put into a queue? We assume that neither computers nor bridges have queues. (3 points)
Answer: 8 times
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