Networks Revision Notes

Internet as a Network of Networks

The Internet is not a single entity but a vast, interconnected global system of computer networks. It allows devices worldwide to communicate and share resources, forming a complex web rather than a centralised hub. This distributed nature contributes to its resilience.

The Role of IP Addressing

IP (Internet Protocol) addresses are unique numerical labels assigned to each device connected to a computer network that uses the Internet Protocol for communication. An IP address serves two main functions: host or network interface identification and location addressing. Without unique IP addresses, data packets would not know where to go.

How Routers Direct Traffic

Routers are crucial networking devices that forward data packets between computer networks. They inspect the destination IP address of a data packet and use routing tables to determine the best path for that packet to reach its destination. This process is often described as 'packet switching'.

Packet Switching in Internet Communication

When data is sent over the Internet, it is broken down into smaller units called packets. Each packet includes part of the original data, along with control information like the source and destination IP addresses. Routers handle these individual packets, directing them independently to their destination, where they are reassembled.

Layers of the TCP/IP Model

The TCP/IP model structures network communication into four logical layers, starting from the application level down to the physical network interface. Each layer has specific responsibilities for processing data as it moves through the network. This structured approach allows for flexibility and easier development of network protocols.

Data Flow During Transmission

When data is transmitted, it begins at the application layer. It then descends through the transport, internet, and link layers, with each layer adding a header (encapsulation). Upon reception, the data ascends through the link, internet, and transport layers, with each layer removing its header (decapsulation) before reaching the receiving application. This ensures data integrity and proper routing across diverse networks.

Application Layer Role

The application layer is the top layer, directly interacting with user applications. It provides network services for applications, handling protocols like HTTP for web browsing, FTP for file transfers, and SMTP for email. This layer prepares the user data for transmission by initiating communication requests.

Transport Layer Role

The transport layer manages end-to-end communication between devices. It segments application data into smaller packets, adding source and destination port numbers. TCP (Transmission Control Protocol) ensures reliable, ordered, and error-checked delivery, while UDP (User Datagram Protocol) offers faster, connectionless, and unreliable delivery.

Internet Layer Role

The internet layer is responsible for logical addressing (IP addresses) and routing packets across different networks. It determines the best path for data to travel from the source to the destination, even if they are on different networks. IP (Internet Protocol) is the primary protocol at this layer.

Link Layer Role

The link layer, also known as the network interface layer, handles the physical transmission of data frames over the local network medium. It manages hardware addressing (MAC addresses), error detection, and physical access methods. This layer is responsible for how data is actually placed on and retrieved from the network cable or wireless signal.

Local Area Network (LAN)

A LAN connects network devices in a small geographical area, such as a single room, building, or campus. Devices within a LAN communicate via high-speed connections, often using Ethernet or Wi-Fi. The infrastructure is typically owned and managed by the organisation or individual using it. Examples include a home network, a school's computer lab network, or an office network.

Wide Area Network (WAN)

A WAN extends over a large geographical expanse, connecting multiple LANs together. This can involve linking offices in different cities or countries. WANs use various technologies, including fibre optic cables, satellite links, and leased lines from telecommunication companies. The internet is the most widely known example of a WAN. Due to the vast distances, WAN speeds are generally lower, and costs higher, than those of LANs.

Key Differences: LAN vs. WAN

The primary distinctions lie in geographical scope, ownership, speed, and cost. LANs are confined to small areas, privately owned, offer high speeds, and are relatively low-cost. WANs cover vast distances, often involve multiple service providers, have generally lower effective speeds, and incur higher costs. Security considerations also differ, with LANs having more manageable internal security and WANs requiring more robust solutions for data traversing public infrastructure.

The Role and Need for Protocols

Protocols are established rules that dictate how data is formatted and exchanged between computing devices. They are vital for ensuring different hardware and software can communicate and understand each other, facilitating reliable and ordered data transmission across networks. Without protocols, heterogeneous systems would lack a common language, making networked communication impossible. This includes everything from web browsing to email and file transfers.

Key Network Protocols

Ethernet and Wi-Fi govern local network communication, specifying how data is transmitted over wired and wireless connections. TCP/IP is a suite that forms the backbone of the internet, handling addressing (IP) and reliable data transfer (TCP). HTTP is for web content delivery, while HTTPS adds encryption for secure web browsing. FTP is specifically for transferring files between a client and server.

Key Email Protocols

POP3 (Post Office Protocol version 3) downloads emails from a server to a local device, usually deleting them from the server. SMTP (Simple Mail Transfer Protocol) is used for sending emails between mail servers and from email clients to servers. IMAP (Internet Message Access Protocol) allows users to manage and access emails directly on the mail server, synchronising across multiple devices and leaving messages on the server until explicitly deleted.