Computer Networks - Online Test

Q1. Given the following state table of an FSM with two states A and B, one input and one output: 

If the initial state is A = 0, B = 0, what is the minimum length of an input string which will take the machine to the state A = 0, B = 1 with Output = 1?
Answer : Option A
Explaination / Solution:
No Explaination.


Q2. A layer-4 firewall (a device that can look at all protocol headers up to the transport layer) CANNOT
Answer : Option A
Explaination / Solution:

Since it is a layer 4 firewall it cannot block application layer protocol like HTTP.

Q3. Consider different activities related to email. m1: Send an email from a mail client to a mail server m2: Download an email from mailbox server to a mail client m3: Checking email in a web browser Which is the application level protocol used in each activity?
Answer : Option C
Explaination / Solution:

Sending an email will be done through user agent and message transfer agent by SMTP, downloading an email from mail box is done through POP, checking email in a web browser is done through HTTP

Q4. HTML (Hyper Text Markup Language) has language elements which permit certain actions other than describing the structure of the web document. Which one of the following actions is NOT supported by pure HTML (without any server or client side scripting) pages?
Answer : Option D
Explaination / Solution:
No Explaination.


Q5. In the IPv4 addressing format, the number of networks allowed under Class C addresses is
Answer : Option C
Explaination / Solution:

For class C address, size of network field is 24 bits. But first 3 bits are fixed as 110; hence total number of networks possible is 221

Q6. Which of the following transport layer protocols is used to support electronic mail?
Answer : Option C
Explaination / Solution:

E-mail uses SMTP, application layer protocol which intern uses TCP transport layer protocol.

Q7. The protocol data unit (PDU) for the application layer in the Internet stack is
Answer : Option C
Explaination / Solution:

The PDU for Datalink layer, Network layer , Transport layer and Application layer are frame, datagram, segment and message respectively.

Q8. Consider a source computer (S) transmitting a file of size 106 bits to a destination computer (D) over a network of two routers (R1 and R2) and three links (L1, L2, and L3). L1 connects S to R1; L2 connects R1 to R2; and L3 connects R2 to D. Let each link be of length 100km. Assume signals travel over each line at a speed of 108 meters per second. Assume that the link bandwidth on each link is 1Mbps. Let the file be broken down into 1000 packets each of size 1000 bits. Find the total sum of transmission and propagation delays in transmitting the file from S to D?
Answer : Option A
Explaination / Solution:
No Explaination.


Q9. An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: 245.248.128.0/20. The ISP wants to give half of this chunk of addresses to Organization A, and a quarter to Organization B, while retaining the remaining with itself. Which of the following is a valid allocation of address to A and B?
Answer : Option A
Explaination / Solution:


Since half of 4096 host addresses must be given to organization A, we can set 12th bit to 1 and include that bit into network part of organization A, so the valid allocation of addresses to A is 245.248.136.0/21 
Now for organization B, 12th bit is set to ‘0’ but since we need only half of 2048 addresses, 13th bit can be set to ‘0’ and include that bit into network part of organization B so the valid allocation of addresses to B is 245.248.128.0/22

Q10. Assume that source S and destination D are connected through two intermediate routers labeled R. Determine how many times each packet has to visit the network layer and the data link layer during a transmission from S to D.


Answer : Option C
Explaination / Solution:


From above given diagram, its early visible that packet will visit network layer 4 times, once at each node [S, R, R, D] and packet will visit Data Link layer 6 times. One time at S and one time at D, then two times for each intermediate router R as data link layer is used for link to link communication. 
Once at packet reaches R and goes up from physical –DL-Network and second time when packet coming out of router in order Network – DL- Physical