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▩ OSI / TCP-IP Layer Reference

Click any OSI layer to expand protocols, PDUs, devices, and N10-009 exam tips. Layers map side-by-side to the TCP/IP model.

OSI Model — 7 Layers
7 Application Data ▶
PDU Data
Protocols HTTP, HTTPS, DNS, SMTP, FTP, SSH, DHCP
Devices None (software / application layer)
Functions
  • User interface to the network
  • Application-level services (email, file transfer, web)
  • Network process to application
N10-009 Exam Tip
DNS is Layer 7 (application) but uses UDP port 53 at Layer 4. Know both layers for DNS on the exam.
6 Presentation Data ▶
PDU Data
Protocols SSL/TLS, JPEG, ASCII, EBCDIC
Devices None
Functions
  • Data formatting, translation, and syntax
  • Encryption and decryption
  • Compression
N10-009 Exam Tip
TLS/SSL encryption happens at Layer 6, not Layer 4. A common distractor on the exam.
5 Session Data ▶
PDU Data
Protocols NetBIOS, RPC, SQL
Devices None
Functions
  • Establishes, manages, and terminates sessions
  • Dialog control (half-duplex / full-duplex)
  • Synchronization and checkpointing
N10-009 Exam Tip
Think of Layer 5 as a phone call — someone sets it up, you talk, then you hang up. Session = setup/teardown.
4 Transport Segment / Datagram ▶
PDU Segment (TCP)  /  Datagram (UDP)
Protocols TCP, UDP
Devices None (implemented in OS)
Functions
  • End-to-end delivery and port numbering
  • Reliability and flow control (TCP)
  • Speed over reliability (UDP)
N10-009 Exam Tip
TCP = reliable (3-way handshake: SYN, SYN-ACK, ACK). UDP = fast, connectionless. Both use port numbers at Layer 4.
3 Network Packet ▶
PDU Packet
Protocols IP (IPv4/IPv6), ICMP, OSPF, BGP
Devices Routers, Layer-3 switches
Functions
  • Logical (IP) addressing
  • Routing and path determination
  • Packet forwarding between networks
N10-009 Exam Tip
Ping uses ICMP — Layer 3. Traceroute also operates at Layer 3. Routers are always Layer 3 devices.
2 Data Link Frame ▶
PDU Frame
Protocols Ethernet, Wi-Fi (802.11), PPP, ARP
Devices Switches, Bridges
Functions
  • MAC (hardware) addressing
  • Error detection via FCS/CRC
  • Framing — encapsulates packets into frames
N10-009 Exam Tip
Switches use MAC address tables — Layer 2. ARP resolves IP to MAC and lives at Layer 2 (some say 2/3 boundary).
1 Physical Bits ▶
PDU Bits
Protocols Ethernet (physical), Wi-Fi (physical signaling)
Devices Hubs, Repeaters, Cables, NICs
Functions
  • Transmits raw bits over physical medium
  • Defines voltage levels, timing, and connectors
  • Cable types: UTP, fiber, coax
N10-009 Exam Tip
Hubs are Layer 1 — they broadcast to all ports with no intelligence. Switches are Layer 2 and use MAC tables.
TCP/IP Model — 4 Layers
Application
Maps to OSI L5 Session + L6 Presentation + L7 Application
HTTP · HTTPS · DNS · FTP · SSH · SMTP · DHCP · TLS · NetBIOS · RPC
Transport
Maps to OSI L4 Transport
TCP · UDP
Internet
Maps to OSI L3 Network
IP · ICMP · OSPF · BGP
Network Access
Maps to OSI L1 Physical + L2 Data Link
Ethernet · Wi-Fi (802.11) · PPP · ARP · Hubs · Switches · Cables

▩ OSI Layer Mnemonics

Bottom to Top (L1 → L7)

Please Do Not Throw Sausage Pizza Away
PPhysical — L1
DData Link — L2
NNetwork — L3
TTransport — L4
SSession — L5
PPresentation — L6
AApplication — L7

Top to Bottom (L7 → L1)

All People Seem To Need Data Processing
AApplication — L7
PPresentation — L6
SSession — L5
TTransport — L4
NNetwork — L3
DData Link — L2
PPhysical — L1