🔌 Network Fundamentals
1
OSI & TCP/IP Reference Models
The OSI Model (7 Layers)
| Layer | Name | Key Protocols/Devices |
|---|---|---|
| 7 | Application | HTTP, HTTPS, FTP, SMTP, DNS |
| 6 | Presentation | TLS, SSL, JPEG, ASCII |
| 5 | Session | NetBIOS, RPC |
| 4 | Transport | TCP, UDP; Segments |
| 3 | Network | IP, ICMP, Routers; Packets |
| 2 | Data Link | Ethernet, MAC; Switches; Frames |
| 1 | Physical | Cables, Hubs, Repeaters; Bits |
TCP/IP Model (4 Layers)
Application → Transport → Internet → Network Access. Maps roughly to OSI layers 7-5, 4, 3, and 2-1 respectively.
2
Network Devices & Topologies
Key Network Devices
- Hub — Layer 1; broadcasts all frames to all ports; half-duplex; obsolete.
- Switch — Layer 2; learns MAC addresses; forwards frames only to the correct port; full-duplex; creates separate collision domains per port.
- Router — Layer 3; routes packets between different networks using IP addresses; separates broadcast domains.
- Firewall — inspects and filters traffic based on rules; can be stateful or stateless.
- Wireless Access Point (WAP) — connects wireless clients to a wired network.
Ethernet Cable Types
- Straight-through — connects different device types (PC to switch, switch to router).
- Crossover — connects same device types (PC to PC, switch to switch). Modern switches use Auto-MDIX to detect automatically.
- Fibre Optic — uses light; immune to EMI; longer distances. Single-mode for long distances; multi-mode for shorter distances.
Network Topologies
- Star — all devices connect to a central switch (most common in LANs).
- Bus — all devices share a single cable (legacy).
- Ring — devices connect in a loop; Token Ring (legacy).
- Mesh — every device connected to every other device; high redundancy.