CIS 456-102, Spring 2006, Dr Ott Name on every sheet! Quiz, 02/17/2006. Keep margin free! Be clear and concise, and within the constraints of clear and concise, be short. 6:00 - 6:40. Class resumes at 6:45. Break around 8.00 Class until ~ 9:25. 1. Draw, in detail, the header of an IPv4 packet without options. For each field give the size in bits, give a SHORT description of its meaning and use. Where applicable, mention what the units are it is expressed in. (e.g., bits, Bytes, meters, miles, coulombs, seconds, etc.) Some students refuse to give units. TL: Total length of the packet (IPv4 header plus IPv4 data) in Bytes. FragOffset: Total number of data bytes in preceding fragments of the same original packet. Expressed in multiples of 8 Bytes. 2. Router R0 has the following forwarding table (kept small, therefore unrealistic): Route Mask Network Interface NextHop 1 255.255.252.0 128.235.204.0 eth0 R4 2 255.255.224.0 128.195.192.0 eth3 R5 3 255.255.240.0 128.195.208.0 eth4 R6 4 255.255.255.255 128.235.204.90 eth2 DD 5 255.255.248.0 128.195.224.0 eth1 DD 6 0.0.0.0 0.0.0.0 eth2 R3 Identify Routes by Route Number, not in any other way unless necessary to prevent confusion. a. Which route is a default route? b. Which route is a host-specific route? c. Give the lengths of all masks. (Identify by route) d. Which routes are prefixes to which other routes? e. For each of the following destination addresses, which route would R0 put the packet on, assuming no problems with TTL, MTU, etc. i 128.235.204.91 ii 128.235.207.90 iii 128.144.204.90 iv 128.195.223.1 v 128.195.204.2 vi 128.235.204.90 vii 128.195.231.3 viii 128.195.163.4 a: Route 6 b: Route 4 (ONLY 128.235.204.90 can use this route). c: 22 , 19 , 20 , 32 , 21 , 0 . d: Several students seem not to know how ``prefixing'' works. Some students seem to think that if two routes have unequal length mask, then the shorter mask is a prefix to the longer. 6 is a prefix to all other routes. 1 is a prefix to 4. 2 is a prefix to 3. Several students said that either 2 or 3 is a prefix to 5, Not true. (Write out only the ``critical'' octet in binary, x is ``anything''): 2: 128.195. 110 twigle x xxxx . anything 3: 128.195. 1101 twigle xxxx . anything 5: 128.195. 1110 0 twigle xxx . anything ^ route 2 as well as route 3 have the 19-th bit different from the 19-th bit of route 5. So neither can be a prefix to route 5. route 2 has mask length 19, and routes 2 and 3 have the same first 19 bits, therefore, route 2 is a prefix to route 3. e i: 1 ii: 1 iii: 6 (because of second octet) iv: 3 (routes 3 and 2 both fit, route 3 has (or is) the longer prefix) v: 2 vi: 4 vii: 5 viii: 6 Grade distribution (100 is perfect): 100 , 94 , 87 , 86 , 85 , 79 , 71 , 69 , 62 , 62 , 61 , 61 , 61 , 38 .