CS 118 Spring 2011 : Homework 5Problem 1Suppose you have a datagram network that uses 32-bit host addresses. Further suppose a router in thisnetwork has four links (0-3) and packets are to be forwarded on these links according to the table below.Dst Range Link11100000 00000000 00000000 00000000through 011100000 11111111 11111111 1111111111100001 00000000 00000000 00000000through 111100001 00000000 11111111 1111111111100001 00000001 00000000 00000000through 211100001 11111111 11111111 11111111otherwise 3a. Provide a forwarding table that has 4 entries, uses longest-prefix matching, and forwards packets tothe correct link interfaces. (Hint: represent the Dst Range in the prefix format, like 11100000 . . . )b. Based on your table, describe for the following destination addresses, which table entry they wouldmatch and what would be the forwarding link interface:11001000 10010001 01010001 0101010111100001 00000000 11000011 0011110011100001 10000000 00010001 0111011111100000 10111011 10100000 00000001a.Prefix Match Link Interface11100000 011100001 00000000 111100001 2otherwise 3b. Prefix match for first address is 4th entry: link interface 3Prefix match for second address is 2nd entry: link interface 1Prefix match for third address is 3rd entry: link interface 2Prefix match for fourth address is 1st entry: link interface 0Problem 2You have a router that connects 3 subnets (1-3) a prefix 223.1.17.0/24. If subnet 1 must support up to 125hosts and subnets 2 and 3 must support up to 60 hosts, then provide network addresses for each subnet (inthe form of a.b.c.d/x) such that these constraints are satisfied.We can divide the /24 prefix into /25, /26, /26 sub-blocks, for example:subnet 1: 223.1.17.0/25subnet 2: 223.1.17.128/26subnet 3: 223.1.17.192/26Page 1 of 2CS 118 Spring 2011 : Homework 5Problem 3What will happen if traceroute is used to find the path to an unassigned address? What is the differenceif the network portion or only the host portion is unassigned?Traceroute sends packets with limited TTL values. If we send to an unassigned network, then as long as thepackets follow default routes, traceroute will get normal answers. When the packet reaches a default-free(backbone) router, however (or more precisely the router which firstrecognizes that the destination doesn’texist), the process will abruptly stop. Packets will not be forwarded further. The router that finally realizesthe error will send back “ICMP host unreachable” or “ICMP net unreachable” based on the unassignedportion.Page 2 of
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