OSI Layer Basics

OSI Model (Quick, Practical View)

The OSI model is just a way to slice networking into layers. Mental map when troubleshooting: start low, move up.

Layer 1: Physical

The raw signal level. Cables, fiber, radio, voltages, light pulses.

• Media: copper, fiber, wireless
• Signal encoding, timing, modulation
• Hubs, repeaters, PHY chips

Layer 2: Data Link

Local network delivery. Frames, MAC addresses, and how devices share the medium.

• Ethernet, Wi‑Fi (802.11)
• MAC addresses (48‑bit)
• Switches live here

802.11 splits L2 into:

• LLC (Logical Link Control)
• MAC (Media Access Control)

Access methods:

• Wired: CSMA/CD
• Wireless: CSMA/CA (can’t listen while transmitting)

MAC address structure:

• First 3 bytes: OUI (vendor)
• Last 3 bytes: device ID

Layer 3: Network

Routing between networks. Packets, IP addresses, and path selection.

Common protocols:

• IP (v4/v6)
• ARP
• ICMP
• IGMP
• IPsec
• Routing protocols (OSPF, BGP, RIP)

Quick notes:

• IPv4 = 32 bits, IPv6 = 128 bits
• ARP maps IP → MAC on local networks
• ICMP is for errors and diagnostics (ping, traceroute)
• IGMP handles multicast group membership
• IPsec provides encryption and authentication at L3

Layer 4: Transport

End‑to‑end delivery. Ports, reliability, and flow control.

• TCP: reliable, ordered, 3‑way handshake
• UDP: lightweight, no guarantee, low overhead
• QoS can be implemented here

Layer 5: Session

Manages conversations between hosts.

• Establish, maintain, and close sessions
• Checkpointing and recovery
• Less visible today, but still a useful concept

Layer 6: Presentation

Makes data readable by the application.

• Encoding/decoding
• Encryption/decryption
• Compression

Layer 7: Application

Where applications live and speak.

• HTTP, FTP, SMTP, DNS, SSH, etc.
• Auth and user‑level interaction
• The part humans usually see