Ch11.10.1: Network Basics

The OSI Model (Open Systems Interconnection)

The OSI model is a conceptual framework that standardizes the functions of a communication system into seven abstraction layers:

Layer	Name	Description	Examples
7	Application	High-level protocols for user applications	HTTP, FTP, SMTP, DNS, SSH
6	Presentation	Data representation, encryption, compression	SSL/TLS, JPEG, MPEG
5	Session	Managing sessions between applications	NetBIOS, RPC
4	Transport	Reliable data transfer between hosts	TCP, UDP
3	Network	Routing and forwarding of packets	IP, ICMP, ARP
2	Data Link	Node-to-node data transfer	Ethernet, Wi-Fi, PPP
1	Physical	Physical transmission of raw bits	Ethernet cables, fiber optics, radio waves

In practice, the TCP/IP model (a simpler 4-layer model) is more commonly used:

Application Layer (combines OSI layers 5–7): HTTP, FTP, DNS, SMTP
Transport Layer: TCP, UDP
Internet Layer: IP, ICMP
Network Access Layer (combines OSI layers 1–2): Ethernet, Wi-Fi

IP Addresses: IPv4 and IPv6

An IP address uniquely identifies a device on a network. There are two versions:

IPv4

32-bit address, written as four decimal numbers separated by dots
Example: 192.168.1.1, 8.8.8.8
Address space: ~4.3 billion addresses (2³²)
Running out of addresses due to Internet growth
Special ranges:
- 127.0.0.1 — localhost (loopback)
- 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — private networks
- 0.0.0.0 — any address
- 255.255.255.255 — broadcast

IPv6

128-bit address, written as eight groups of four hexadecimal digits separated by colons
Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Can omit leading zeros and consecutive zero groups: 2001:db8:85a3::8a2e:370:7334
Address space: ~3.4×10³⁸ addresses (enough for every atom on Earth)
Special addresses:
- ::1 — localhost (loopback)
- :: — any address
- fe80::/10 — link-local addresses

Common Ports

A port is a 16-bit number (0–65535) that identifies a specific service on a host. Well-known ports (0–1023) are assigned by IANA:

Port	Protocol	Service
20–21	TCP	FTP (File Transfer Protocol)
22	TCP	SSH (Secure Shell)
23	TCP	Telnet (unencrypted, avoid)
25	TCP	SMTP (Simple Mail Transfer Protocol)
53	TCP/UDP	DNS (Domain Name System)
67–68	UDP	DHCP (Dynamic Host Configuration Protocol)
80	TCP	HTTP (Hypertext Transfer Protocol)
110	TCP	POP3 (Post Office Protocol v3)
143	TCP	IMAP (Internet Message Access Protocol)
443	TCP	HTTPS (HTTP Secure)
465	TCP	SMTPS (SMTP over SSL)
587	TCP	SMTP (submission, with STARTTLS)
993	TCP	IMAPS (IMAP over SSL)
995	TCP	POP3S (POP3 over SSL)
3306	TCP	MySQL
3389	TCP	RDP (Remote Desktop Protocol)
5432	TCP	PostgreSQL
5900	TCP	VNC (Virtual Network Computing)
6379	TCP	Redis
8080	TCP	HTTP alternate (often used for proxies)
8443	TCP	HTTPS alternate
27017	TCP	MongoDB

Note: Ports 0–1023 are well-known ports (require root/admin to bind). Ports 1024–49151 are registered ports. Ports 49152–65535 are dynamic/ephemeral ports (used for client-side connections).