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1、本试卷共 5 页，第 1 页 东 北 大 学 考 试 试 卷 （ 测 验 二 ） 2015 2016 学年 第 一 学期 课程名称：计算机网络（Computer Networks） 总分 一 二 三 四 五 六 七 八 学 院 班 级 学 号 姓 名 密 封 线 1. Addressing at the network and link layers, routing versus switching. Consider the network shown below. Each of the subnets A-D contains at most 31 hosts; subnet E co
2、nnects routers R1 and R2. a. Assign network addresses to the five subnets shown above (that is, write down the addresses you have assigned). b. Assign (write down) a full (32-bit) IP address for each the two hosts shown in subnets A and D. c. Assign (write down) a full IP address to the router inter
3、face on subnet E. d. What is the network prefix advertised by router R1 to the public Internet? e. Assign (write down) a MAC address to D. f. Does the host in A ever need to know the MAC address of the R1s interface in subnet E in order to send an IP packet to the host in D? Explain your answer in o
4、ne or two sentences. Now suppose that router R2 above is replaced by an Ethernet switch, S2 (Router R1 remains a router). g. Are the interfaces that previously were in subnets A, B, and E still in the same separate three IP subnets now that R2 is replaced by S2? Explain your answer in a few sentence
5、s. h. In order to send an IP packet to the host in D, does the host in A ever need to know the MAC address of the R1s left interface now that R2 is replaced by S2? If so, how does it get the MAC address of R1s left interface? Explain your answer in one or two sentences. Answer: a. Each subnet needs
6、to address up to 31 hosts, using the rightmost 5 bits of the address. The five subnet addresses are thus x.y.z.000_/27, x.y.z.001_/27, x.y.z.010_/27, x.y.z.011_/27, x.y.z.100_/27, where the notation x.y.z.000_ means that the leftmost three bits of the fourth address byte are 000. Other answers with
7、different bit values in bits 25, 26, 27 are also possible, as long as the five three-bit patterns used are unique. b. If you chose an address range x.y.z.000_/27 for network A, then the address you choose here must have these 27 leading bits, and can have any 5 remaining bits you want. If you chose
8、an address range x.y.z.011_/27 for network D, then the address you choose here must have these 27 leading bits, and can have any 5 remaining bits you want. c. If you chose an address range x.y.z.100_/27 for network E, then the address you choose here must have these 27 leading bits, and can have any
9、 5 remaining bits you want. d. x.y.x.24 e. Any 48 bit number is OK. f. No. The host in subnet A needs to address a link-layer frame (containing the IP packet addresses to the host in D) to the R2 interface in subnet A only. g. No. They are now all in the same subnet from an IP addressing point of vi
10、ew, since there is no longer any intervening router. h. Yes. Now the host in A now needs to address its link-layer frame to the left interface of R1. The host in A gets the MAC address of the left interface of R1 using ARP. The host in A knows that in order to route its packet to the host in D, it m
11、ust first send that packet (over Ethernet) to router R1, whose IP address is in the hosts routing table. Thus, it uses ARP to get the MAC address associated with the IP address of R1s left interface. 本试卷共 5 页，第 2 页 学 院 班 级 学 号 姓 名 密 封 线 b. The shortest path from E to B is E F B. The cost of this pat
12、h is 6. c. The shortest path from B to D is B A C D. The cost of this path is 5. 2. Dijkstras (link-state) algorithm. (13.14.15) Consider the network shown below. a. Show the operation of Dijkstras (link-state) algorithm for computing the least cost path from D to all destinations. What is the short
13、est path from D to B, and what is the cost of this path? b. Show the operation of Dijkstras (link-state) algorithm for computing the least cost path from E to all destinations. What is the shortest path from E to B, and what is the cost of this path? c. Show the operation of Dijkstras (link-state) a
14、lgorithm for computing the least cost path from B to all destinations. What is the shortest path from B to D, and what is the cost of this path? Answer: a. The shortest path from D to B is D C A B. The cost of this path is 5. 本试卷共 5 页，第 3 页 学 院 班 级 学 号 姓 名 密 封 线 b. From its neighbors, nodes A, B, an
15、d D. Note that C does not receive distance vectors from nodes E and F, since they are not direct neighbors. c. See page 358 in the textbook for notation. Cs cost to E via B is c(C,B) + DB(E) = 8 + 6 = 14. Cs cost to E via A is c(C,A) + DA(E) = 2 + 7 = 9 (note that As shortest path to E is through C!) Cs cost to E via D is c(C,D) + DD(E) = 1 + 4 = 5 Thus, C will route to E via D, since that path through D has minimum cost. d. From its neighbors, nodes B
