COE 431 Final Exam Problems, Fall 2013

Problem I: IP subnetting (10 minutes) 1. If a host on a network has the address 172.16.45.14/30, what is the address of the subnetwork to which the host belongs? a. 172.16.45.0 b. 172.16.45.4 c. 172.16.45.8 d. 172.16.45.12 e. 172.16.45.18 2. You have a class C network, and you need ten subnets. You wish to have as many addresses available for hosts as possible. Which one of the following subnet masks should you use? a. 255.255.255.192 b. 255.255.255.224 c. 255.255.255.240 d. 255.255.255.248 e. None of the above 3. Which two of the addresses below are available for host addresses on the subnet 192.168.15.19/28 (actually, this is the IP address of a host that belongs to this subnet)? (select two answer choices) a. 192.168.15.17 b. 192.168.15.14 c. 192.168.15.29 d. 192.168.15.31 e. None of the above 4. The IP network 210.106.14.0 is subnetted using a /24 mask. How many usable networks and host addresses can be obtained from this? a. 1 network with 254 hosts b. 4 networks with 128 hosts c. 2 networks with 24 hosts d. 6 networks with 64 hosts e. None of the above 5. Given that you have a class B IP address network range, which of the subnet masks below will allow for 100 subnets and 500 usable host addresses per subnet? a. 255.255.0.0 b. 255.255.224.0 c. 255.255.254.0 d. 255.255.255.0 e. None of the above 1 Problem II: Routing (10 minutes) 1. An IP router (which does longest prefix match to route packets) has the following forwarding table mapping prefixes to outgoing interfaces: 128.2.0.0/24  interface 1 128.2.1.0/24  interface 2 128.2.2.0/24  interface 2 128.2.3.0/24  interface 3 But space in the forwarding table might be scarce. Suppose, for example, my router only has space for 3 prefixes in its forwarding table. Produce a forwarding table with only 3 entries which is equivalent to the one above – that is, each IP address will still be mapped to same outgoing interface as in the above 4-entry table. Assume the router uses the longest prefix matching to match incoming packets to interfaces. 128.2.0.0/24  interface 1 128.2.0.0/22  interface 2 128.2.3.0/24  interface 3 2. You are given the following network prefixes: 171.64.121.0/24 171.64.180.0/24 171.64.18.0/24 171.64.181.0/24 171.64.19.0/24 171.64.187.0/24 You are to aggregate the prefixes together into the smallest possible number of shorter prefixes. What is the smallest number of prefixes and what are they? a. 1 prefix: 171.64.179.0/21 b. 2 prefixes: 171.64.180.0/20, 171.64.18.0/21 c. 4 prefixes: 171.64.180.0/23, 171.64.187.0/24, 171.64.121.0/24, 171.64.18.0/23 d. 3 prefixes: 171.64.180.0/20, 171.64.187.0/24, 171.64.18.0/23 2 Problem III: Routing protocols (10 minutes) 1. Consider the following network topology that we denote by G. a. Find the shortest path from node A to each other node in G. b. Populate the table given below with the entries found in A’s routing table? More specifically, for each destination give the neighbor to be used by A to get to that destination. Destination Next hop B B C D D D E D F D 3 Problem IV: TCP’s congestion window (15 minutes) Assume a host running TCP Reno (which came after TCP Tahoe) is connected to a link with maximum segment size (MSS) of 2.5 Kbytes. The host initializes CongWin to 1 segment and Threshold to 16 segments. The round trip time RTT on this link is 50 ms. 1. What is the value of CongWin after 300 msec? a. 16 MSS b. 15 MSS c. 17 MSS d. Other (give your answer): 18 MSS 2. Assume a packet is sent at time = 300 msec. This packet is lost and the loss event is detected via receipt of triple duplicate ACKs. What is the new value of CongWin? a. 1 MSS b. 8 MSS c. 4 MSS d. Other (give your answer): 9 MSS 3. Assume a packet is sent at time=300 msec, is lost and a timeout occurs. What is the new value of Threshold? a. 9 MSS b. 8 MSS c. 1 MSS d. Other (provide your answer): 4. Assuming a first packet loss is detected after the 9th transmission round via a timeout event, what will be the values of: The congestion window: a. 1 MSS b. 8 MSS c. 11 MSS d. Other (put your answer): The threshold: a. 8 MSS b. 9 MSS c. 11 MSS d. Other (put your answer): 10 MSS 4