Computer Networks

Layer 2 of the OSI model is the Data Link Layer, responsible for node-to-node data transfer and physical addressing (MAC addresses). Layer 1 is Physical (electrical/signaling), Layer 3 is Network (routing, IP addressing), and Layer 4 is Transport (end-to-end communication like TCP/UDP).

Layer 4 of the OSI model is the Transport Layer. It provides end-to-end communication services, including error recovery and flow control. Layers from bottom to top: Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), Application (7).

The Transport Layer (Layer 4) in TCP/IP handles reliability (through TCP acknowledgments and retransmissions), flow control (managing data transmission rates), and error correction. TCP provides these services, while UDP is a simpler protocol without them.

The TCP/IP Application Layer combines the top 3 OSI layers: Application (Layer 7), Presentation (Layer 6), and Session (Layer 5). The Session layer establishes, manages, and terminates connections between applications.

IP (Internet Protocol) is the universal protocol that provides the addressing and routing mechanism for all internet communication. Every device on the internet has an IP address, enabling global connectivity. TCP, HTTP, and FTP all build on top of IP.

The Application Layer (Layer 7) is the top OSI layer that directly interfaces with user applications outside the OSI model. It provides network services like file transfer, email, and web browsing to applications. Lower layers (Physical, Network) handle technical transmission aspects, while Presentation handles data formatting. The Application Layer is the only layer that applications directly interact with.

The OSI reference model consists of exactly 7 layers: Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), and Application (7). This 7-layer architecture is a fundamental concept in networking. Options A (2), B (3), and C (5) are all incorrect - they don't represent the complete OSI model structure.

Layer 3 of the OSI reference model is the Network Layer. This layer handles routing, logical addressing (IP addresses), and path determination between networks. It's responsible for forwarding packets to their destination across multiple networks. The Physical layer is Layer 1, Transport is Layer 4, and Data Link is Layer 2 - so options A, B, and C are incorrect.

Layer 1 of the OSI reference model is the Physical Layer. This layer deals with the physical connection between devices and transmission of raw bit streams over the network medium. It specifies electrical, mechanical, and procedural requirements for activating and deactivating physical connections. Transport is Layer 4, Data Link is Layer 2, and Network is Layer 3.

In layered architecture, each layer communicates with its peer layer on another device. The data unit exchanged between peer layers is called a Protocol Data Unit (PDU). Different layers have different PDU names - segments at transport layer, packets at network layer, frames at data link layer - but collectively they're all PDUs.

DNS servers map hostnames to IP addresses, so hackers can use DNS data to discover a host's network location. TCP/IP, Telnet, and SSL servers don't provide this network mapping information.

IP addresses have 5 classes: Class A, B, C for standard networking; Class D for multicasting; and Class E for experimental/reserved use. This is a fundamental IP addressing concept. Option C is correct.

The loopback address 127.0.0.1 is the standard IPv4 address used for testing the local network stack. It refers to the local machine and is used for diagnostics and local services. This is a fundamental networking concept. Option A is correct.

Routers operate at Layer 3 (Network Layer) of the OSI model, where they make forwarding decisions based on IP addresses. They route packets between different networks by examining destination IP addresses and using routing tables to determine the best path. Switches operate at the Data Link Layer, not routers.

Switches operate at Layer 2 (Data Link Layer) of the OSI model, using MAC addresses to forward frames within a network segment. They maintain MAC address tables to learn which devices are connected to which ports, enabling efficient frame forwarding. Routers operate at the Network Layer, not switches.