The Internet enables users to access numerous services and computers around the world. These services rely on standardised protocols from the TCP/IP suite and operate at the application layer of the OSI model. Each service uses one or more well-defined TCP/UDP ports.

1. Email

Email has been the most widely used Internet tool since its inception. It allows users to communicate asynchronously. Messages can be sent to individuals or groups via mailing lists. The key feature of email is that it is addressed to a specific user (identified by their address: utilisateur@domaine.tld) on a specific host.

Architecture of a messaging system:

Sending protocols: SMTP (Simple Mail Transfer Protocol, port 25/587/465) handles the transfer of emails between servers and from the client to the outgoing server. STARTTLS and SMTPS encrypt the communication.

Receiving protocols: POP3 (Post Office Protocol 3, port 110/995) downloads messages and deletes them from the server. IMAP4 (Internet Message Access Protocol, port 143/993) synchronises messages with the server, allowing access from multiple devices.

Email security: SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail) and DMARC help combat identity theft (spoofing) and spam. S/MIME and PGP encrypt and sign the content of messages.

2. DNS — Domain Name System

The DNS (Domain Name System) is the Internet’s name resolution system. It translates human-readable domain names (www.example.com) into IP addresses that computers can use (93.184.216.34). Without the DNS, you would have to memorise the IP addresses of all the websites you visit.

Hierarchical DNS architecture:

Root servers: 13 clusters of root servers (a.root-servers.net to m.root-servers.net) know the authoritative servers for each TLD (Top-Level Domain).

TLD servers (Top-Level Domain): manage domain extensions (.com, .fr, .org, .net). AFNIC manages the .fr domain.

Authoritative servers: hold the DNS records for a specific domain (A, AAAA, MX, CNAME, TXT, NS, SOA, etc.).

Recursive resolvers: provided by ISPs or third-party services (Google 8.8.8.8, Cloudflare 1.1.1.1). They make queries on behalf of clients and cache the results.

⚡ DNSSEC, DoH and DoT

DNSSEC (DNS Security Extensions) adds cryptographic signatures to DNS responses to prevent cache poisoning attacks (cache poisoning). DoH (DNS over HTTPS, RFC 8484) and DoT (DNS over TLS, RFC 7858) encrypt DNS queries to protect users’ privacy from their ISPs. These protocols are supported by Firefox, Chrome, Windows 11 and modern public resolvers.

3. World Wide Web (HTTP/HTTPS)

The World Wide Web (invented by Tim Berners-Lee in 1991) is the most visible Internet service. It is based on three fundamental technologies:

HTML (HyperText Markup Language): a language for describing web pages, which structures the content (text, images, forms, links). HTML5 (2014) added native multimedia capabilities (audio, video, 2D/3D canvas, WebSockets).

URL (Uniform Resource Locator): a unique address identifying a resource on the web (format: protocol://host:port/path?parameters).

HTTP/HTTPS (HyperText Transfer Protocol): a protocol for transferring web pages (port 80 for HTTP, port 443 for HTTPS). HTTP/1.1 is the historical standard; HTTP/2 (2015) improves performance through request multiplexing; HTTP/3 (2022) uses the QUIC protocol (over UDP) to reduce latency.

Security via TLS (Transport Layer Security) is now mandatory (HTTPS) for all business websites: it ensures confidentiality (encryption), data integrity and server authentication via X.509 certificates (Let's Encrypt, DigiCert).

4. FTP / SFTP / FTPS

FTP (File Transfer Protocol, ports 20/21) enables the transfer of files between a client and a remote server. Access can be anonymous (public FTP) or password authenticated. FTP transmits data in plain text — not recommended on the Internet.

SFTP (SSH File Transfer Protocol): file transfer via an encrypted SSH tunnel (port 22). Recommended for any transfer of sensitive data.

FTPS (FTP Secure): FTP with TLS encryption (default port 990 or explicit port 21). Requires certificate management on the server side.

⚠ FTP — an outdated protocol for secure file transfers

FTP should no longer be used to transfer sensitive files over the internet, as usernames and passwords are transmitted in plain text. SFTP or FTPS should be used instead. For general file sharing, cloud services (Dropbox, Google Drive, OneDrive, Nextcloud) have largely replaced FTP.

 

5. Telnet and SSH

Telnet (port 23) allows you to log in remotely to a host computer as a dumb terminal, in order to access its files and run its programmes. Telnet transmits all data (including passwords) in plain text.

SSH (Secure Shell, port 22): completely replaces Telnet with strong encryption (AES, ChaCha20), public/private key authentication (RSA, Ed25519) and data integrity (HMAC). SSH also supports port tunnelling (port forwarding), file transfer (SFTP/SCP) and the creation of VPN tunnels. It is the standard for the remote administration of Linux/Unix servers and network equipment.

⚠ Telnet — obsolete and dangerous

Telnet should never again be used for system administration on an unsecured network. SSH has completely replaced Telnet since the 2000s. Modern network equipment (switches, routers) disables Telnet by default and only accepts SSH.

 

6. Cloud and streaming services

Internet usage has shifted towards highly centralised cloud services:

Cloud computing (IaaS/PaaS/SaaS): AWS (Amazon Web Services), Microsoft Azure and Google Cloud Platform host the majority of the world’s web services. The cloud offers scalability, availability and economies of scale. By 2024, over 60% of global internet traffic will pass through cloud infrastructure.

Video streaming: Netflix (>250 million subscribers), YouTube (>2 billion users), Disney+, Amazon Prime Video. Video streaming accounts for around 65% of global internet traffic. CDNs (Content Delivery Networks — Akamai, Cloudflare, Fastly) distribute content from servers close to the user to reduce latency.

Video conferencing: Zoom, Microsoft Teams and Google Meet have seen explosive growth since 2020. They use WebRTC (Web Real-Time Communication) protocols for browser-to-browser peer-to-peer communication without the need for plugins.

IoT and APIs: the Internet of Things is generating increasing volumes of sensor data (MQTT, CoAP, HTTP/REST). REST APIs (JSON/HTTP) and GraphQL enable interconnection between web services. Over 9 billion connected devices by 2024.

7. Usenet — Newsgroups

Usenet is one of the first online discussion systems to emerge, predating the World Wide Web (1979). It allows users to join thousands of discussion groups (newsgroups) organised by topic, using the NNTP (Network News Transfer Protocol, port 119). Usenet was very popular in the 1990s for technical and academic discussions. Today, it has largely been replaced by web forums, Reddit, Discord and social media.

Modifié le: vendredi 9 octobre 2026, 09:57