All questions of Network Layer and Internetworking for Computer Science Engineering (CSE) Exam

Header Fields of an IP Packet

An IP (Internet Protocol) packet consists of a header followed by the data payload. The header contains various fields that provide information about the packet and help in routing it across the network. These fields are typically modified by IP routers as the packet is forwarded from the source to the destination.

Fields Modified by a Typical IP Router

1. Checksum: The IP header includes a checksum field which is used to detect errors in the packet during transmission. The checksum is calculated by the sender and verified by each router along the path. If errors are detected, the packet is discarded. Routers may update the checksum field if modifications are made to the IP header.

2. Source Address: The source address field in the IP header specifies the IP address of the sender. As the packet is forwarded through different routers, the source address remains unchanged. It is an essential field as it helps in identifying the origin of the packet.

3. Time to Live (TTL): The TTL field in the IP header indicates the maximum number of hops or routers the packet can traverse before being discarded. Each router that processes the packet decrements the TTL value by 1. If the TTL reaches 0, the packet is dropped. Routers must update the TTL field as they forward the packet to ensure its proper handling.

Field NOT Modified by a Typical IP Router

- Length: The length field in the IP header specifies the total length of the IP packet, including both the header and the data payload. This field is set by the sender and remains unchanged throughout the routing process. Routers do not modify the length field as they forward the packet.

Conclusion

In summary, among the given options, the field that is NOT modified by a typical IP router is the source address. The checksum, TTL, and length fields may be updated or modified by routers as they process and forward the IP packet.

The "route" command is used to manipulate the TCP/IP routing table.

The TCP/IP routing table is a critical component of networking that helps determine the path that network traffic takes from one network to another. It contains a list of routes, each specifying a destination network and the next-hop IP address or interface through which the traffic should be forwarded.

The "route" command is a command-line utility that allows users to view and manipulate the routing table on a computer. It is available on most operating systems, including Windows, Linux, and macOS.

Usage of the "route" command:
The "route" command can be used with various options and parameters to perform different tasks. Some of the commonly used options include:

1. View the routing table: The command "route print" displays the current routing table on the system. It provides information such as the destination network, subnet mask, gateway, interface, and metric.

2. Add a new route: The command "route add" is used to add a new route to the routing table. It requires specifying the destination network, subnet mask, gateway, and interface. For example, "route add 192.168.1.0 mask 255.255.255.0 192.168.0.1" adds a route for the network 192.168.1.0/24 using the gateway 192.168.0.1.

3. Delete an existing route: The command "route delete" is used to remove a route from the routing table. It requires specifying the destination network and subnet mask. For example, "route delete 192.168.1.0 mask 255.255.255.0" deletes the route for the network 192.168.1.0/24.

4. Modify an existing route: The command "route change" is used to modify the properties of an existing route. It requires specifying the destination network, subnet mask, gateway, and interface. For example, "route change 192.168.1.0 mask 255.255.255.0 192.168.0.2" changes the gateway for the route to 192.168.0.2.

Summary:
In summary, the "route" command is a powerful tool used to manipulate the TCP/IP routing table. It allows users to view, add, delete, and modify routes in the routing table, which determines how network traffic is routed.

Explanation:

Virtual Circuit Network is a type of network in which a dedicated path is established between the source and destination before transmitting the data. In this network, each packet contains a short VC (Virtual Circuit) number instead of a full source and destination address.

Advantages of using VC number:
- It reduces the overhead of routing and forwarding packets by eliminating the need for address lookup in each router along the path.
- It provides faster and more efficient packet switching as the network can quickly route the packets based on the VC number.

Disadvantages of using VC number:
- It requires a lot of resources to establish and maintain the virtual circuit path, which can result in higher latency and delay in transmission.
- It can also lead to network congestion as multiple virtual circuits may share the same physical link.

Therefore, the correct answer to the question is option 'B', which states that each packet in a virtual circuit network contains a short VC number instead of a full source and destination address.

C

Algorithms for effectively routing messages from a source to multiple destination nodes in a store-and-forward computer network are studied. The focus is on minimizing the network cost (NC), which is the sum of weights of the links in the routing path.

Explanation:

To determine the valid subnet address of the given IP address, we need to determine the network portion of the IP address. The subnet mask for the given IP address is /25, which means the first 25 bits of the IP address are used for the network portion, and the remaining 7 bits are used for the host portion.

The valid subnet address is obtained by setting all the host bits to zero. Therefore, we need to determine the network address that corresponds to the given IP address.

The given IP address is 172.16.112.1/25. To determine the network address, we need to perform a logical AND operation between the IP address and the subnet mask:

IP address: 172.16.112.1
Subnet mask: 255.255.255.128 (/25)

Performing a logical AND operation between the IP address and the subnet mask gives us:

Network address: 172.16.112.0

Therefore, the valid subnet address for the given IP address is 172.16.112.0.

HTML-formatted answer:

• IP address: 172.16.112.1


• Subnet mask: 255.255.255.128 (/25)

• Network address: 172.16.112.0

MIB (Management Information Base)

The Management Information Base (MIB) is a collection of groups of objects that can be managed using the Simple Network Management Protocol (SNMP). It is a key component in network management systems, allowing administrators to monitor and control network devices.

SNMP (Simple Network Management Protocol)

SNMP is an application layer protocol that is used to manage and monitor network devices. It provides a way for network administrators to manage network devices, such as routers, switches, and servers, by retrieving and setting information about these devices. SNMP operates over the User Datagram Protocol (UDP) or Transmission Control Protocol/Internet Protocol (TCP/IP) network protocols.

How MIB and SNMP work together

MIB is a hierarchical database that organizes information about managed objects in a tree-like structure. Each node in the tree represents an object, and the branches represent relationships between objects. The MIB defines the structure and attributes of each object, allowing SNMP to retrieve or set values for these objects.

SNMP uses a manager-agent model, where the SNMP manager is responsible for collecting and monitoring information, and the SNMP agent is responsible for providing the information to the manager. The SNMP manager communicates with the SNMP agent using SNMP messages. These messages are used to retrieve information from the MIB, set values for objects, and receive traps or notifications from the agent.

Advantages of using SNMP and MIB

- Standardized protocol: SNMP is an industry-standard protocol that is supported by a wide range of network devices and management systems. This allows for interoperability between different vendors' devices and management systems.

- Scalability: SNMP and MIB provide a scalable solution for network management. The hierarchical structure of the MIB allows for easy organization and management of large networks with thousands of devices.

- Flexibility: SNMP allows for the monitoring and control of a wide range of network devices and parameters. The MIB can be extended or customized to include additional objects specific to a particular network or device.

- Real-time monitoring: SNMP provides real-time monitoring of network devices, allowing administrators to proactively identify and resolve issues before they impact network performance.

- Troubleshooting and diagnostics: SNMP and MIB provide valuable information for troubleshooting and diagnostics. By retrieving information from the MIB, administrators can analyze network performance, identify bottlenecks, and diagnose problems.

Conclusion

In conclusion, the correct answer is option 'C' SNMP. MIB is a collection of groups of objects that can be managed using SNMP. SNMP allows for the monitoring and control of network devices by retrieving and setting information from the MIB. This combination provides a powerful and flexible solution for network management.

The correct answer is option 'B', a network layer firewall works as a packet filter. Let's understand why:

Packet Filter:
A packet filter is a type of firewall that operates at the network layer (layer 3) of the OSI model. It examines the source and destination IP addresses, source and destination ports, and other parameters of each packet passing through it. Based on a set of predefined rules, the packet filter determines whether to allow or block the packet from passing through.

Working of a Network Layer Firewall:
A network layer firewall examines each packet that passes through it and applies a set of rules to determine whether to allow or block it. It does this by analyzing the packet header information, including source and destination IP addresses, source and destination port numbers, and other protocol-specific parameters.

Advantages of a Network Layer Firewall:
1. Granular Control: A network layer firewall provides granular control over network traffic by allowing administrators to define specific rules for packet filtering. This allows organizations to enforce their security policies and restrict access to specific resources or services.

2. Improved Performance: Since a network layer firewall operates at the network layer, it can process packets more efficiently compared to higher-layer firewalls. It filters packets based on their header information, making it faster and more scalable for handling large amounts of network traffic.

3. Protocol Independence: A network layer firewall is protocol-independent, meaning it can filter packets for various protocols such as TCP, UDP, ICMP, etc. This allows organizations to protect their networks from different types of attacks and vulnerabilities.

4. Flexibility: Network layer firewalls can be implemented using dedicated hardware appliances or software-based solutions. This provides flexibility in choosing the right firewall solution based on the organization's requirements and budget.

Conclusion:
In summary, a network layer firewall works as a packet filter by examining the header information of each packet and applying a set of rules to determine whether to allow or block it. It provides granular control, improved performance, protocol independence, and flexibility in protecting the network from unauthorized access and potential threats.

Explanation:

CSMA/CD Access Method:
CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It is a network protocol that allows multiple devices to share the same communication channel efficiently by listening to the channel, checking for traffic, and transmitting data when the channel is clear. In case of a collision, the devices involved detect the collision and follow a backoff algorithm to retransmit their data.

Which Architecture uses CSMA/CD Access Method?
The architecture that uses CSMA/CD access method is Ethernet.

Ethernet:
Ethernet is a widely used LAN technology that employs CSMA/CD as its access method. In Ethernet networks, devices listen to the communication channel before transmitting data. If the channel is clear, they go ahead and transmit their data. However, if a collision is detected, they use CSMA/CD to handle the collision and retransmit the data after a random backoff period.

Other Options:
- ARCnet: ARCnet uses a token-passing protocol instead of CSMA/CD.
- Router: A router is a networking device that forwards data packets between computer networks. It does not use CSMA/CD.
- STP Server: STP (Spanning Tree Protocol) is used to prevent loops in network topologies and does not use CSMA/CD for access control.
Therefore, the correct option is Ethernet when it comes to architectures that use CSMA/CD access method.

Explanation:
- An IP address is a 32-bit number that uniquely identifies a device on the internet.
- The IP address is divided into two parts: network address and host address.
- The network address is used to identify the network, while the host address is used to identify the device on that network.
- The IP address is divided into classes, and each class has a different number of network and host bits.
- The most commonly used classes are A, B, and C.
- Class A has 8 bits for network and 24 bits for host.
- Class B has 16 bits for network and 16 bits for host.
- Class C has 24 bits for network and 8 bits for host.
- An ISP that requests a block of 1000 addresses would require a Class C network, which has 256 possible network addresses and 256 possible host addresses.
- Therefore, the ISP would be granted 4 blocks of Class C networks, which is equal to 1024 addresses (4 x 256).
- Hence, the correct answer is option 'C' (1024).

Explanation:

Dijkstra's Algorithm:
- OSPF (Open Shortest Path First) uses Dijkstra's algorithm to compute the shortest path.
- Dijkstra's algorithm is a graph search algorithm that solves the single-source shortest path problem for a graph with non-negative edge weights.
- It starts at the source node and explores the neighboring nodes to find the shortest path to all other nodes in the graph.

Bellman-Ford Algorithm:
- The Bellman-Ford algorithm is another algorithm used for finding the shortest path in a network.
- It can handle graphs with negative edge weights but is not typically used in OSPF.

Routing Information Protocol (RIP):
- RIP is another routing protocol that uses distance vector routing, which is different from OSPF.
- It does not use Dijkstra's algorithm for computing the shortest path.

Distance Vector Routing:
- Distance vector routing, used by protocols like RIP, determines the best path based on the number of hops to a destination.
- OSPF, on the other hand, uses Dijkstra's algorithm which considers the actual cost or weight of the paths.
In conclusion, OSPF uses Dijkstra's algorithm for computing the shortest path because it is efficient and suitable for networks with non-negative edge weights.

B) No.

18.14.12.0 is not a valid host address as the last octet (0) represents the network address. The first usable host address for a block of 1000 addresses in the 18.14.12.0 network would be 18.14.12.1, and the last usable host address would be 18.14.15.254.

WiMAX, which stands for Worldwide Interoperability for Microwave Access, is a wireless communication technology that provides high-speed internet access over long distances. It is based on the IEEE 802.16 standard and operates in the microwave frequency range.

Orthogonal Frequency Division Multiplexing (OFDM) is the modulation scheme employed by WiMAX, making option A the correct answer. OFDM is a digital modulation technique that divides the available bandwidth into multiple subcarriers, each of which carries a portion of the data. These subcarriers are orthogonal to each other, meaning they do not interfere with one another, allowing for efficient use of the available spectrum.

Below, we will explain in detail why WiMAX uses OFDM and why the other options are not correct:

Orthogonal Frequency Division Multiplexing (OFDM)
- OFDM is a widely used modulation scheme in wireless communication systems, including WiMAX.
- It divides the available bandwidth into multiple subcarriers, each carrying a portion of the data.
- The subcarriers are orthogonal to each other, meaning they can be transmitted simultaneously without interfering with one another.
- This allows for efficient use of the available spectrum, increasing the overall data throughput.

Time Division Multiplexing (TDM)
- TDM is a different modulation scheme that divides the available bandwidth into time slots.
- Each user or device is assigned a specific time slot to transmit their data.
- While TDM is used in some communication systems, WiMAX does not use it as its primary modulation scheme.

Space Division Multiplexing (SDM)
- SDM is a technique that uses multiple antennas to transmit and receive data simultaneously.
- It takes advantage of the spatial dimension to increase the overall data throughput.
- While SDM is used in some wireless communication systems, WiMAX does not rely on it as its primary modulation scheme.

Conclusion
In summary, WiMAX uses Orthogonal Frequency Division Multiplexing (OFDM) as its primary modulation scheme. OFDM allows for efficient use of the available spectrum by dividing it into multiple subcarriers that can be transmitted simultaneously without interference. This increases the overall data throughput and enables high-speed internet access over long distances.

Explanation:

To determine the correct option, let's analyze the given information about the IP packet:

The first 8 bits of the IP packet are 0100 0010.

IP Version:
The first 4 bits of an IP packet represent the IP version. In this case, the first 4 bits are '0100', which corresponds to IP version 4. Therefore, the IP packet is using IPv4.

Header Length:
The next 4 bits of an IP packet represent the header length. In this case, the next 4 bits are '0010', which corresponds to a header length of 2 words. Each word is 4 bytes. So, the total header length is 2 * 4 bytes = 8 bytes.

Hops:
The number of hops a packet can travel is determined by the Time to Live (TTL) field in the IP header. However, in this case, the first 8 bits do not provide any information about the TTL. Therefore, we cannot determine the number of hops from the given information.

Upper Layer Protocol:
The upper layer protocol is determined by the Protocol field in the IP header. However, the first 8 bits do not provide any information about the upper layer protocol. Therefore, we cannot determine the upper layer protocol from the given information.

Receiver's Action:
The receiver's action cannot be determined from the first 8 bits of the IP packet. The receiver may reject the packet for various reasons such as invalid checksum, incorrect IP version, or other errors. However, we cannot determine the receiver's action solely based on the given information.

Therefore, the correct option is 'D) The receiver rejects the packet' since it is the only option that cannot be determined from the given information.