# Recent questions and answers in Computer Networks

1
In the network $200.10.11.144/27$, the $fourth$ octet (in decimal) of the last $IP$ address of the network which can be assigned to a host is _____.
2
$\text{Host A}$ sends a $\text{UDP}$ datagram containing $8880\text{ bytes}$ of user data to $\text{host B}$ over an $\text{Ethernet LAN}.$ Ethernet frames may carry data up to $1500\text{ bytes (i.e. MTU = 1500 bytes)}.$ Size of $\text{UDP}$ ... and what will be the contents of offset field in the last fragment? $6$ and $925$ $6$ and $7400$ $7$ and $1110$ $7$ and $8880$
3
Consider the following routing table at an IP router: $\begin{array}{|l|l|l|} \hline \textbf {Network No} & \textbf {Net Mask} & \textbf{Next Hop} \\\hline \text {128.96.170.0} & \text{255.255.254.0} & \text{Interface$0$} \\\hline\text {128.96.168.0} & \text{255.255.254.0} & \text{Interface$ ... i-a, ii-c, iii-e, iv-d i-a, ii-d, iii-b, iv-e i-b, ii-c, iii-d, iv-e i-b, ii-c, iii-e, iv-d
4
Assume that the bandwidth for a $TCP$ connection is $1048560$ bits/sec. Let $\alpha$ be the value of RTT in milliseconds (rounded off to the nearest integer) after which the $TCP$ window scale option is needed. Let $\beta$ be the maximum possible window size with window scale option. Then ... $^{16}$ $500$ milliseconds, $65535$ $\times$2$^{14}$ $500$ milliseconds, $65535$ $\times$2$^{16}$
5
An $IP$ router with a $\text{Maximum Transmission Unit (MTU)}$ of $1500$ bytes has received an $IP$ packet of size $4404\text{ bytes}$ with an $IP$ header of length $20\text{ bytes}$. The values of the relevant fields in the header of the third $IP$ fragment generated by the ... $1,$ Datagram Length$: 1500;$ Offset$: 370$ $\text{MF bit}$: $0,$ Datagram Length$: 1424;$ Offset$: 2960$
6
An IP router implementing Classless Inter-domain Routing (CIDR) receives a packet with address $131.23.151.76$ ... The identifier of the output interface on which this packet will be forwarded is ______.
7
Consider the store and forward packet switched network given below. Assume that the bandwidth of each link is $10^6$ bytes / sec. A user on host $A$ sends a file of size $10^3$ bytes to host $B$ through routers $R1$ and $R2$ in three different ways. In the first case a single ... third case respectively. Which one of the following is CORRECT? $T1<T2<T3$ $T1>T2>T3$ $T2=T3, T3<T1$ $T1=T3, T3> T2$
8
Consider a selective repeat sliding window protocol that uses a frame size of $1$ $\text{KB}$ to send data on a $1.5$ $\text{Mbps}$ link with a one-way latency of $50$ $\text{msec}$. To achieve a link utilization of $60\%$, the minimum number of bits required to represent the sequence number field is ________.
9
Let the size of congestion window of a TCP connection be $32$ KB when a timeout occurs. The round trip time of the connection is $100$ msec and the maximum segment size used is $2$ KB. The time taken (in msec) by the TCP connection to get back to $32$ KB congestion window is _________.
10
Two popular routing algorithms are Distance Vector(DV) and Link State (LS) routing. Which of the following are true? (S1): Count to infinity is a problem only with DV and not LS routing (S2): In LS, the shortest path algorithm is run only at one node (S3): In DV, the ... ): DV requires lesser number of network messages than LS S1, S2 and S4 only S1, S3 and S4 only S2 and S3 only S1 and S4 only
11
In an IPv4 datagram, the $M$ bit is $0$, the value of $HLEN$ is $10$, the value of total length is $400$ and the fragment offset value is $300$. The position of the datagram, the sequence numbers of the first and the last bytes of the payload, respectively are: Last fragment, $2400$ and $2789$ First fragment, $2400$ and $2759$ Last fragment, $2400$ and $2759$ Middle fragment, $300$ and $689$
12
Using public key cryptography, $X$ adds a digital signature σ to message $M$, encrypts $<M, \sigma>$, and sends it to $Y$, where it is decrypted. Which one of the following sequences of keys is used for the operations? Encryption: $X's$ private key followed by $Y's$ ... key; Encryption: $X's$ private key followed by $Y's$ public key; Decryption: $Y's$ private key followed by $X's$ public key
13
Consider a network with five nodes, $N1$ to $N5$, as shown as below. The network uses a Distance Vector Routing protocol. Once the routes have been stabilized, the distance vectors at different nodes are as follows. N1: $(0, 1, 7, 8, 4)$ N2: $(1, 0, 6, 7, 3)$ N3: $(7, 6, 0, 2, 6)$ N4: $(8, 7, 2, 0, 4)$ ... at node, $N3$? $(3, 2, 0, 2, 5)$ $(3, 2, 0, 2, 6)$ $(7, 2, 0, 2, 5)$ $(7, 2, 0, 2, 6)$
14
Consider a network with $6$ routers $R1$ to $R6$ connected with links having weights as shown in the following diagram. All the routers use the distance vector based routing algorithm to update their routing tables. Each router starts with its routing table initialized to contain an entry ... tables stabilize, how many links in the network will never be used for carrying any data? $4$ $3$ $2$ $1$
15
Suppose computers $A$ and $B$ have $IP$ addresses $10.105.1.113$ and $10.105.1.91$ respectively and they both use same netmask $N$. Which of the values of $N$ given below should not be used if $A$ and $B$ should belong to the same network? $255.255.255.0$ $255.255.255.128$ $255.255.255.192$ $255.255.255.224$
16
If a class $B$ network on the Internet has a subnet mask of $255.255.248.0$, what is the maximum number of hosts per subnet? $1022$ $1023$ $2046$ $2047$
17
A client process P needs to make a TCP connection to a server process S. Consider the following situation: the server process S executes a $\text{socket()}$, a $\text{bind()}$ and a $\text{listen()}$ system call in that order, following which it is preempted. ... $\text{connect()}$ system call returns an error $\text{connect()}$ system call results in a core dump
18
Which of the following system calls results in the sending of SYN packets? socket bind listen connect
19
What is the maximum size of data that the application layer can pass on to the TCP layer below? Any size $2^{16}$ bytes - size of TCP header $2^{16}$ bytes $1500$ bytes
20
Host $A$ is sending data to host $B$ over a full duplex link. $A$ and $B$ are using the sliding window protocol for flow control. The send and receive window sizes are $5$ packets each. Data packets (sent only from $A$ to $B$) are all $1000$ bytes long and the transmission time for ... communication? $7.69 \times 10^6$ Bps $11.11 \times 10^6$ Bps $12.33 \times 10^6$ Bps $15.00 \times 10^6$ Bps
21
The address of a class $B$ host is to be split into subnets with a $6$-$bit$ subnet number. What is the maximum number of subnets and the maximum number of hosts in each subnet? $62$ subnets and $262142$ hosts. $64$ subnets and $262142$ hosts. $62$ subnets and $1022$ hosts. $64$ subnets and $1024$ hosts.
22
Consider a $TCP$ connection in a state where there are no outstanding $ACK$s. The sender sends two segments back to back. The sequence numbers of the first and second segments are $230$ and $290$ respectively. The first segment was lost, but the second segment was received correctly by the receiver. Let ... $Y$ (in that order) are $60$ and $290$ $230$ and $291$ $60$ and $231$ $60$ and $230$
23
Two computers $C1$ and $C2$ are configured as follows. $C1$ has IP address $203.197.2.53$ and netmask $255.255.128.0$. $C2$ has IP address $203.197.75.201$ and netmask $255.255.192.0$. Which one of the following statements is true? $C1$ and $C2$ both assume ... assumes $C2$ is on same network, but $C2$ assumes $C1$ is on a different network $C1$ and $C2$ both assume they are on different networks.
24
Suppose that it takes $1$ unit of time to transmit a packet (of fixed size) on a communication link. The link layer uses a window flow control protocol with a window size of $N$ packets. Each packet causes an ack or a nak to be generated by the receiver, and ack/nak transmission times are negligible. Further, ... is $1- \dfrac{ N}{i}$ $\dfrac{i}{(N + i)}$ $1$ $1 - e^{\left(\frac{i}{N}\right)}$
25
Consider three IP networks $A, B$ and $C$. Host $H_A$ in network $A$ sends messages each containing $180$ $bytes$ of application data to a host $H_C$ in network $C$. The TCP layer prefixes $20$ byte header to the message. This passes through an intermediate network $B$. The maximum packet ... other overheads. $325.5$ $\text{Kbps}$ $354.5$ $\text{Kbps}$ $409.6$ $\text{Kbps}$ $512.0$ $\text{Kbps}$
26
Suppose that the maximum transmit window size for a TCP connection is $12000$ $\text{bytes}$. Each packet consists of $2000$ $\text{bytes}$. At some point in time, the connection is in slow-start phase with a current transmit window of $4000$ $\text{bytes}$. Subsequently, the ... the current transmit window? $4000$ $\text{bytes}$ $8000$ $\text{bytes}$ $10000$ $\text{bytes}$ $12000$ $\text{bytes}$
27
A sender is employing public key cryptography to send a secret message to a receiver. Which one of the following statements is TRUE? Sender encrypts using receiver's public key Sender encrypts using his own public key Receiver decrypts using sender's public key Receiver decrypts using his own public key
28
Suppose that in an IP-over-Ethernet network, a machine X wishes to find the MAC address of another machine Y in its subnet. Which one of the following techniques can be used for this? X sends an ARP request packet to the local gateway's IP address which ... ARP request packet with broadcast MAC address in its local subnet X sends an ARP request packet with broadcast IP address in its local subnet
29
Consider an IP packet with a length of $4,500$ $bytes$ that includes a $20-byte$ IPv4 header ans $40-byte$ TCP header. The packet is forwarded to an IPv4 router that supports a Maximum Transmission Unit (MTU) of $600$ $bytes$. Assume that the ... Assume that the fragmentation offset value stored in the first fragment is $0$. The fragmentation offset value stored in the third fragment is _____.
30
Given a mask, M=255.255.255.248. How many subnet bits are required for given mask M? (A) 2 (B) 3 (C) 4 (D)5
31
In an RSA cryptosystem, the value of the public modulus parameter $n$ is $3007$. If it is also known as that $\phi(n)=2880$ where $\phi()$ denotes Euler’s Totient Function, then the prime factor of $n$ which is greater than $50$ is _________
32
The three way handshake for TCP connection establishment is shown below. Which of the following statements are TRUE? $S1:$ Loss of $SYN + ACK$ from the server will not establish a connection $S2:$ Loss of $ACK$ from the client cannot establish the connection $S3:$ ... machine on no packet loss $S2$ and $S3$ only $S1$ and $S4$ only $S1$ and $S3$ only $S2$ and $S4$ only
33
In the $4B/5B$ encoding scheme, every $4$ bits of data are encoded in a $5$-bit codeword. It is required that the codewords have at most $1$ leading and at most $1$ trailing zero. How many are such codewords possible? $14$ $16$ $18$ $20$
34
Match the following: ... $\text{P-II, Q-I, R-IV, S-III}$ $\text{P-IV, Q-I, R-II, S-III}$ $\text{P-IV, Q-I, R-III, S-II}$
35
A broadcast channel has $10$ nodes and total capacity of $10$ Mbps. It uses polling for medium access. Once a node finishes transmission, there is a polling delay of $80$ μs to poll the next node. Whenever a node is polled, it is allowed to transmit a maximum of $1000$ bytes. The maximum throughput of the broadcast channel is: $1$ Mbps $100/11$ Mbps $10$ Mbps $100$ Mbps
36
Consider the following statements about the functionality of an IP based router. A router does not modify the IP packets during forwarding. It is not necessary for a router to implement any routing protocol. A router should reassemble IP fragments if the MTU of the outgoing link is ... the incoming IP packet. Which of the above statements is/are TRUE? I and II only I only II and III only II only
37
The following data fragment occurs in the middle of a data stream for which the bytestuffing algorithm described in the text is used: A B ESC C ESC FLAG FLAG D. What is the output after stuffing
Two hosts are connected via a packet switch with $10^7$ bits per second links. Each link has a propagation delay of $20$ microseconds. The switch begins forwarding a packet $35$ microseconds after it receives the same. If $10000$ bits of data are to be ... time elapsed between the transmission of the first bit of data and the reception of the last bit of the data in microseconds is ______.