IT 420
Spring 2014
Homework 6

1. An IP packet is 4000 bytes long and carries 3980 bytes of payload data. What values will be in the HLEN (also called IHL) and Total Length fields?

2. A large IPv4 datagram carrying 2300 bytes of payload data is fragmented to pass over a network where the MTU is 820 bytes. Assume the IP header is 20 bytes in each fragment. For each fragment, show the contents of the Identification field, the Offset field, the Total Length field, and the More Fragments flag in the IP header. (Choose any value for the identification fields.)

3. A large IPv4 datagram is fragmented into 3 fragments at router 1 to pass over a network with an MTU of 1500 bytes. Assume each fragment is larger than 900 bytes. Then, each fragment arrives at router 2, which wants to forward the fragments over a network with an MTU of 900 bytes. How many fragments will arrive at the destination host?

4. If necessary, read the instructions on Moodle about how to connect to an osl machine here at NJIT. Then, connect to osl60.njit.edu and perform the following. Type “netstat -rn” to display the routing table in this host. What is the IP address of the default router for osl60.njit.edu? You may use another host if osl60 is down – just tell me which one you are using.

5. A router R has forwarding table:

128.235.223.0/24 deliver directly
128.235.176.0/20 128.235.184.1
128.235.168.0/21 128.235.170.1
128.235.176.0/21 128.235.178.1
128.235.172.0/22 128.235.172.1
0.0.0.0/0 128.235.178.1

Explain what router R will do with packets with the following destination addresses. Make sure to use longest match routing.

a. 128.235.169.82
b. 128.235.179.25
c. 128.235.223.38
d. 128.235.184.7

6. Consider the diagram below, which uses unrealistic IP and MAC addresses to simplify the problem:

a. Assuming the ARP tables are up to date, what are the source and destination IP addresses and the source and destination MAC addresses in the frame that goes from the source host to the router?

b. Assuming the ARP tables are up to date, what are the source and destination IP addresses and the source and destination MAC addresses in the frame that goes from the router to the destination host?

c. Assume the ARP table in the source host is empty, so it will have to ARP to find the MAC address of the Router A connection. What are the source and destination MAC addresses in the ARP request? What are the source and destination addresses in the ARP response?

d. Assume the router has been replaced by a switch, which is not assigned any IP addresses or MAC addresses. Going from the source host to the destination host is now one hop, not two as in the previous questions. What are the source and destination IP addresses and the source and destination MAC addresses in the frame that goes from the source host to the destination host? What does the switch do with the frame?

7. Explain how CIDR, NAT, and DHCP each conserve IP addresses.

8. Using the Wireshark IP trace provided on Moodle, answer the following questions:

a. Examine packet 8, a ping request. What is the ICMP type field? What are the values of all of the fields in the IP header?
b. Examine packet 9, an ICMP response. What is the ICMP type field?
c. Examine packets 92 and 93, which are two fragments. How can you tell from the headers that packet 92 is the first fragment and packet 93 is the second fragment?