1 Networking Fundamentals and Architectures

Network foundations and scope

A computer network is a group of connected devices that exchange data according to shared rules called protocols. End-user devices—including computers, phones, printers, and servers—are hosts. Links carry signals between hosts and network devices using wired media such as copper or fiber, or wireless signals.

Networks combine local connections with routers and other links to connect separate networks. Their operation depends on both the physical connections and the protocols shared by communicating devices.

Network reach and service architectures

Networks are often classified by geographic reach. A local area network (LAN) connects devices in a limited area, such as a home, office, or campus, and may use Ethernet, Wi-Fi, or both. A wide area network (WAN) connects networks over larger distances. The Internet is a global network of interconnected networks. A metropolitan area network (MAN) spans a city or metropolitan region, although its classification can overlap with WANs.

Networks also differ in how they organize services. In a client-server architecture, clients request services from servers; for example, a browser can request a webpage. In a peer-to-peer architecture, devices can communicate or share resources directly without relying on one central server for every exchange. Real networks may combine these patterns.

Layers and network protocols

Communication follows layered protocols. The TCP/IP model provides a practical way to understand the work involved: applications create data; transport protocols carry it between applications; IP addresses and routes move packets between networks; and link technologies deliver data across each local connection.

TCP and UDP are transport protocols. TCP provides reliable, ordered delivery, while UDP offers a connectionless transport service. IPv4 and IPv6 are versions of IP used for network addressing and packet delivery.

Network devices and their roles

A switch connects devices within a LAN and forwards Ethernet frames toward the appropriate local destination. A router connects different IP networks, examines a packet’s destination IP address, and forwards it toward the next network using its routing information.

A wireless access point lets wireless hosts join a local network. A modem or another access device may connect a home or organization to an Internet service provider. These devices have distinct roles: switching delivers traffic across a local link or LAN, while routing carries packets between IP networks. A home gateway often combines several functions in one device.

Physical and logical topologies

Topology describes how network devices and links are arranged. Physical topology is the actual layout, while logical topology describes how data flows. They do not always match: Ethernet devices, for example, may be physically connected in a star around a switch while traffic is logically exchanged between the sending and receiving devices.

Common arrangements include:

• Point-to-point: One link joins two endpoints.

• Star: Each endpoint connects to a central switch or hub. This is common in wired LANs. A central-device failure can affect many connections, while failure of one endpoint link usually affects only that endpoint.

• Bus: Devices share a main communication line. A failure or disruption on that line can affect the whole segment.

• Ring: Devices form a loop, and data travels around the ring according to the network’s method of operation.

• Mesh: Devices have multiple interconnections. A full mesh connects every node to every other node; a partial mesh provides selected alternate paths, improving resilience at added cost and complexity.

• Tree or hierarchical: Multiple star segments connect in levels, allowing networks to expand while organizing links.

Networks commonly combine topologies. For example, a campus design might use star-shaped access networks connected in a hierarchy.

How data travels between hosts

When an application sends data, each layer adds information needed by the next stage. In this process, called encapsulation, application data is carried by a transport segment or datagram, placed in an IP packet, and then carried in a link-layer frame. At the destination, the layers process and remove those headers to deliver the data to the receiving application.

Consider a laptop requesting a webpage from a server:

1. The browser creates a request. A transport protocol such as TCP or UDP carries application data between the communicating applications. TCP provides reliable, ordered delivery; UDP offers a connectionless transport service.

2. The laptop compares the server’s IP address with its own local network information. If the server is on the same IP network, the laptop sends the frame across the local link toward the server. If the server is on another network, the laptop sends the frame to its default gateway, usually a router.

3. The local switch forwards the frame through the LAN. The router removes the incoming link-layer frame, examines the IP packet, and forwards it in a new frame on the next link. A packet may pass through several routers. The link-layer frame can change at each network hop, while the destination IP address guides routing.

4. The destination host receives the packet, processes the headers in reverse order, and delivers the request to the appropriate application. The server’s response travels back through the network.

IP provides packet delivery across interconnected networks, but IP itself does not guarantee that every packet arrives. When reliability is needed, it is provided by protocols such as TCP.