The 7-layer OSI model in one clear, exam-ready reference: each layer's name, function, key protocols, and devices — plus how data flows down the stack from application to physical. A must-have for Network+ and CCNA candidates. More free study resources at bytebarhq.com.
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7 — Application
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User-facing services. Protocols: HTTP, FTP, SMTP, DNS, DHCP
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6 — Presentation
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Data format, encryption, compression. Protocols: TLS/SSL, JPEG, ASCII
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5 — Session
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Connections between apps. Protocols: NetBIOS, RPC, PPTP
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4 — Transport
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End-to-end delivery, ports. Protocols: TCP, UDP. Devices: Firewall (stateful)
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3 — Network
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Routing, logical addressing. Protocols: IP, ICMP, OSPF, BGP. Devices: Router, L3 switch
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2 — Data Link
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Switch-to-switch, MAC addressing. Protocols: Ethernet, ARP, PPP. Devices: Switch, bridge, NIC
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1 — Physical
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Bits on the wire. Devices: Hub, repeater, cables
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Top → bottom
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All People Seem To Need Data Processing
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Bottom → top
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Please Do Not Throw Sausage Pizza Away
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4 Transport
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Segment (TCP) / Datagram (UDP)
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3 Network
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Packet
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2 Data Link
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Frame
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1 Physical
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Bits
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Application
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7 + 6 + 5 (Application, Presentation, Session)
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Transport
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4 (Transport)
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Internet
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3 (Network)
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Network Access
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2 + 1 (Data Link, Physical)
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ARP
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Works at Layer 2/3 boundary (resolves IP → MAC)
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TLS
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Lives at Layer 6 (Presentation)
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Switches vs Routers
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Switches = Layer 2 (MAC addresses) · Routers = Layer 3 (IP addresses)
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MTU
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MTU issues and fragmentation → Layer 3
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