A computer network consists of two or more nodes (devices such as computers, phones, or servers) connected by links (cables or wireless signals) so they can exchange data. Networks provide the underlying infrastructure for modern technologies like web browsing, email, video calls, and cloud services.

Networks are commonly categorized by geographic scope:
* **Local Area Network (LAN):** Connects devices within a single location, such as a home or office.
* **Wide Area Network (WAN):** Connects networks spread across large geographic distances. The Internet is the largest WAN in existence.

The Internet is not a single centralized entity, but rather a "network of networks" consisting of millions of independent networks that communicate using shared protocols—standardized rules for formatting and exchanging data.

Computer Networks and Scale

Because networking is complex, communication tasks are divided into modular layers where each layer performs a specific job and relies on the layer beneath it. This separation allows individual protocols and technologies to evolve independently.

The theoretical benchmark is the seven-layer **OSI Model**:
1. **Physical:** Transmits raw physical signals, cables, radio waves, and voltages.
2. **Data Link:** Moves data between directly connected devices using MAC addresses.
3. **Network:** Routes packets across multiple networks using IP addresses.
4. **Transport:** Manages end-to-end communication between programs (e.g., TCP and UDP).
5. **Session:** Manages ongoing communication sessions between applications.
6. **Presentation:** Handles data formatting, translation, and encryption.
7. **Application:** Interfaces directly with user-facing software such as browsers and email clients.

In practical Internet implementations, the simplified four-layer **TCP/IP Model** is used, comprising the Link, Internet, Transport, and Application layers. Communication relies on **encapsulation**: as data moves down the protocol stack on the sending device, each layer attaches its own header. Upon arrival at the receiving host, data moves upward through the stack as each header is inspected and stripped away in turn.

Layered Models and Encapsulation

Data delivery across networks relies on distinct addressing mechanisms operating at different layers:

* **MAC Address:** A permanent hardware address burned into a device's network interface card. It uniquely identifies a device within a local network segment.
* **IP Address:** A logical address used to route data across interconnected networks. IPv4 addresses (such as `192.168.1.5`) provide approximately 4 billion unique addresses. Because IPv4 address space became depleted, IPv6 was introduced with much longer addresses to accommodate growing numbers of devices.
* **Port Number:** Identifies a specific program, application, or service running on a host. Standard examples include port 80 for HTTP and port 443 for HTTPS.
* **Domain Name System (DNS):** Functions as the Internet's directory, translating human-readable domain names (such as `example.com`) into machine-usable numerical IP addresses.

Together, an IP address combined with a port number specifies the precise application on a target host that should receive incoming data.

Network Addressing and Identity

Protocols define the rules for exchanging data reliably or efficiently across networks:

* **Internet Protocol (IP):** Operates on a best-effort basis to deliver packets from source to destination using IP addresses, offering no inherent guarantees regarding delivery or packet ordering.
* **Transmission Control Protocol (TCP):** Provides reliable, ordered delivery. TCP numbers data segments, tracks receipts, and retransmits lost packets. It is suited for applications requiring high data accuracy, such as web pages, file transfers, and email.
* **User Datagram Protocol (UDP):** A lightweight, connectionless protocol that sends data without delivery confirmations or ordering guarantees. UDP is preferred where speed is prioritized over lossless transmission, such as live video streams, voice calls, and online gaming.
* **Application & Configuration Protocols:** HTTP and HTTPS govern web page retrieval (HTTPS wraps HTTP inside TLS encryption). DHCP automatically assigns IP addresses and network configuration to devices when they join a network.

Core Transport and Network Protocols

Data is broken down into small units called **packets**, each containing payload data along with source and destination address headers. Data forwarding is carried out by specialized network hardware:

* **Switches:** Connect devices within a local network (LAN) and forward frames directly to target devices using MAC addresses.
* **Routers:** Interconnect different networks and determine the optimal path for packets using IP addresses, routing data hop-by-hop across networks (e.g., connecting a home LAN to the Internet).
* **Access Points:** Enable wireless devices to connect to a wired network via Wi-Fi.
* **Firewalls:** Inspect inbound and outbound traffic against defined security policies to block unauthorized access.

For TCP-based communication, data transfer begins only after completing a **three-way handshake**:
1. The client sends a `SYN` (synchronize) message.
2. The server responds with a `SYN-ACK` message to acknowledge.
3. The client replies with an `ACK` message, establishing the connection.

Once the connection is established, data packets travel independently across routers and are reassembled in proper order at the destination.

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