Virginia Tech

Department of Computer Science

CS4254 Computer Network Architecture and Programming

Spring 2006


Homework 2 (Total 50 pts)

Due in class on Thursday February 9th, 2006


Please hand in a hardcopy of your homework in a folder, which has your name and course number, in class on Thursday Feb. 9th, 2006. (Always keep a copy of the homework you hand in). Each solution must include all calculations and an explanation of why the given solution is correct. In particular, write complete sentences. A correct answer without an explanation is worth no credit. No late homework will be accepted. Electronic preparation of your solutions is mandatory.


Q1. (10 pts) Consider two hosts, Hosts A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.

a)     Express the propagation delay, dprop in terms of m and s.

b)     Determine the transmission time of the packet, dtrans in terms of L and R.

c)     Ignoring processing and queuing delays, obtain an expression for the end-to-end delay.

d)     Suppose Host A begins to transmit the packet at time t=0. At time t=dtrans, where is the last bit of the packet?

e)     Suppose dprop is greater than dtrans . At time t=dtrans, where is the first bit of the packet?

f)      Suppose dprop is less than dtrans . At time t=dtrans, where is the first bit of the packet?

g)     Suppose s=2.5*108, L=100 bits and R=28 kbps. Find the distance m so that dprop equals dtrans.


Q2. (5 pts) A subnet mask in class A has fourteen 1s. How many subnets does it define?


Q3. (10 pts) “On a network that uses NAT, an external host can initiate communication with an internal host (behind the NAT router)”. Comment on the validity of this statement explaining your answer.


Q4. (5 pts) Suppose there are 3 routers between source and destination hosts. Ignoring fragmentation, an IP segment sent from source host to destination host will travel over how many interfaces? How many routing tables will be indexed to move the datagram from source to destination?


Q5. (10 pts) In a class B subnet, we know the IP address of one of the hosts and the mask as given below

IP address:



What is the subnet address?


Q6. (10 pts) Consider sending a 3000 byte datagram into a link that has a MTU of 500 bytes over an IPv4 network. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are their characteristics (specifically, what are the values for the following fields in the IP header of every fragment: total length, identification, MF flag, and offset)?