What is a computer network? Explain different network topologies with diagrams. Compare TCP/IP model with OSI model.

A computer network is a group of two or more interconnected computing devices that can share data, resources, and communicate with each other.

Network Topologies:

1. Bus Topology — All devices connected to a single central cable (bus/backbone). Data travels in both directions. Simple and inexpensive but failure of main cable stops entire network.

2. Star Topology — All devices connected to a central hub/switch. Most commonly used. If one connection fails, others remain unaffected. But hub failure affects entire network.

3. Ring Topology — Each device connected to exactly two other devices forming a circular path. Data travels in one direction. Failure of one device can disrupt the network.

4. Mesh Topology — Every device connected to every other device. Most reliable and redundant. Expensive due to many cables and ports required.

5. Tree/Hierarchical Topology — Combination of star and bus topologies. Used in large networks with multiple levels.

Comparison of TCP/IP and OSI Models:

Basis	OSI Model	TCP/IP Model
Full Form	Open Systems Interconnection	Transmission Control Protocol/Internet Protocol
Layers	7 layers	4 layers
Developed by	ISO	DARPA (US Department of Defense)
Nature	Theoretical/reference model	Practical/implementation model
Layer structure	Application, Presentation, Session, Transport, Network, Data Link, Physical	Application, Transport, Internet, Network Access
Usage	Used as a teaching and reference framework	Actually used on the Internet