7.1 Client-Server vs Peer-to-Peer (P2P) Architecture
Network applications generally follow either a centralized Client-Server topology or a decentralized Peer-to-Peer (P2P) paradigm.
Figure 7.1: Client-Server concentrates data centrally; Peer-to-Peer distributes load symmetrically across all participants.
7.2 DNS — The Domain Name System
The Domain Name System (DNS) is a global, hierarchical distributed database running on UDP port 53 that resolves human-readable domain names (e.g. www.google.com) into 32-bit or 128-bit IP addresses.
Figure 7.2: Recursive DNS resolution sequence querying Root, Top-Level Domain (TLD), and Authoritative servers.
7.3 HTTP and HTTPS (Hypertext Transfer Protocol)
HTTP is a stateless, application-level request-response protocol running over TCP port 80. HTTPS encapsulates HTTP inside TLS/SSL encryption running over TCP port 443.
Figure 7.3: The standard HTTP stateless client-server request and response sequence.
7.4 Email Protocols (SMTP, POP3, IMAP), FTP, and DHCP
Core infrastructure protocols powering everyday Internet operations:
- SMTP (Simple Mail Transfer Protocol - Port 25): Push protocol used by email clients to send mail to mail servers, and between MTAs.
- POP3 (Post Office Protocol 3 - Port 110) & IMAP (Port 143): Pull protocols used by users to retrieve mail. POP3 downloads and erases from server; IMAP synchronizes folders across multiple client devices.
- FTP (File Transfer Protocol - Ports 20 & 21): Uses two separate connections: Port 21 for Control commands and Port 20 for raw Data transfer.
- DHCP (Dynamic Host Configuration Protocol - Ports 67 & 68): Automatically leases IP addresses and subnet masks to joining devices via the 4-step DORA process (Discover, Offer, Request, Acknowledge).
