CS Free MCQ Bank

[ Option C ]

In the TCP/IP model, the Transport Layer uses protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), where UDP (User Datagram Protocol) is connectionless and lightweight. The Network (Internet) Layer is responsible for routing and error reporting, and uses protocols like ICMP (Internet Control Message Protocol).
Finally, the Application layer supports end-user services such as FTP for file transfer, HTTP for web access, and SMTP for email.

[ Option C ]

The Fragment Offset field in the IPv4 header is 13 bits long. In the IPv4 header, the Fragment Offset field is used to indicate the position of a fragment in the original packet when fragmentation occurs.

• It is measured in units of 8 bytes, allowing the reassembly of fragmented packets.
• This 13-bit field supports offsets from 0 to 8191, which multiplied by 8 equals a maximum byte offset of 65,528.
• This field, together with the More Fragments (MF) flag, helps in fragment reassembly at the destination.

[ Option A ]

The Total Length field in an IPv4 datagram header is 16 bits long. This field specifies the complete size of the IP packet, which includes both the header and the data portion.
Because it is a 16-bit field, it can represent values ranging from a minimum of 20 bytes (when no payload) up to a maximum of 65535 bytes.
 

[ Option D ]

TCP (Transmission Control Protocol) is a connection-oriented protocol, which means it establishes a reliable connection before sending data. The three-way handshake involves three steps, SYN, SYN-ACK, and ACK.

• SYN: Client sends a synchronize (SYN) packet to the server.
• SYN-ACK: Server responds with SYN + acknowledgment (ACK).
• ACK: Client sends back an ACK to the server.

After these three steps, the connection is established and both sides have agreed on initial sequence numbers.

[ Option D ]

ICMP (Internet Control Message Protocol) is a network layer (Layer 3) protocol in the OSI model. It is used for sending error messages and network status information. For example, commands like ping and traceroute use ICMP to check if a device is reachable.

[ Option C ]

In the TCP/IP protocol suite, each TCP connection is uniquely identified by a socket, which consists of four elements:

1. Source IP address.
2. Source port number.
3. Destination IP address.
4. Destination port number.

This combination is called a 4-Tuple. It ensures that even if multiple clients use the same source port number, or a single client opens multiple connections to the same server, the server can distinguish each connection uniquely.

Therefore, even though Host C and Host A both use source port 26145 to connect to Server B’s HTTP port, their source IPs differ, making the 4‑tuples unique. The server’s socket table tracks each connection by its 4‑tuple and can distinguish and route data correctly.

[ Option B ]

In IPv4, fragmentation could occur at both source and intermediate routers, meaning routers could break large packets into smaller fragments to fit the network’s Maximum Transmission Unit (MTU).

The IPv6 changed the approach to make packet handling faster and simpler. In IPv6, only the source node is allowed to perform fragmentation, and reassembly is done only at the destination, not at intermediate routers. 
If a router receives a packet larger than the MTU, it does not fragment the packet. Instead, it drops the packet and sends an ICMPv6 “Packet Too Big” error message back to the sender.

[ Option B ]

In the TCP/IP Protocol Suite, the Transport Layer (OSI Model Layer 4) is responsible for providing end-to-end communication services for applications. Its main functions include reliable data delivery, flow control, error detection, and multiplexing.
TCP, UDP, and SCTP are transport layer protocols in the TCP/IP suite. TCP is reliable and connection-oriented, UDP is connectionless, SCTP is a message-oriented, reliable transport with multi-homing and multi-streaming.
SNMP (Simple Network Management Protocol) is an application layer protocol used for network management.

[ Option B ]

In the TCP header, the HLEN (Header Length) field indicates the length of the TCP header. The minimum header length is 20 bytes when no options are used. The HLEN field is 4 bits long, and its value tells how many 4-byte words are in the header.
Now,
HLEN = 0111 (binary) = 7 (decimal)
So, total TCP header length = 7 × 4 = 28 bytes
Since the standard TCP header (without options) is 20 bytes, the extra part = 28 – 20 = 8 bytes, which corresponds to the options field.

[ Option C ]

The Domain Name System (DNS) is used to convert domain names (www.surakuacademy.com) into IP addresses that computers can understand. DNS mainly communicates over the Internet using two transport layer protocols:

1. UDP (User Datagram Protocol).
2. TCP (Transmission Control Protocol).

For most regular DNS queries, UDP is used because it is fast and efficient. Since DNS requests and responses are usually small (less than 512 bytes), using UDP avoids the extra time needed to establish a connection, making it ideal for quick lookups such as finding the IP address of a website.
DNS also uses TCP in specific cases where reliability is required. When a DNS response exceeds 512 bytes, during zone transfers between servers, or if a UDP query fails, TCP provides a reliable, connection-oriented transfer. 
In short, DNS uses UDP for speed and TCP for reliability and large data transfers, ensuring both efficiency and accuracy.
 

[ Option A ]

In IPv4, a packet may need to be fragmented if it is larger than the Maximum Transmission Unit (MTU) of the network.

Fragmentation: The large packet is broken into smaller fragments.
Reassembly: The receiver combines the fragments back into the original packet.
IPv4 uses two fields in the IP header to manage this process:

• More Fragments (MF) Flag: Indicates if more fragments follow.
• Fragment Offset: Shows the position of the fragment in the original packet.

These fields allow the receiver to correctly reassemble all fragments in order.

[ Option D ]

When an IP packet arrives, the first 8 bits of its header play a crucial role in determining how it is processed. These 8 bits are divided into two fields: the first 4 bits represent the IP version, and the next 4 bits indicate the Internet Header Length (IHL).

In this case, the first 8 bits are 01000010. The first 4 bits, 0100, correspond to version 4, which confirms that this is an IPv4 packet. The next 4 bits, 0010, represent the IHL value of 2.

The IHL specifies the number of 32-bit words in the header. Since each word equals 4 bytes, the total header length here is 2 × 4 = 8 bytes. However, the minimum valid IPv4 header length is 20 bytes (IHL = 5).

Therefore, an IHL value of 2 is invalid and indicates a malformed packet. As a result, the receiver will reject this packet because it does not meet the minimum requirements for processing.