1.10 OSI and TCP/IP Reference Models
Key Takeaways
- The OSI model has 7 layers (Physical, Data Link, Network, Transport, Session, Presentation, Application).
- The TCP/IP model has 4 layers (Network Interface, Internet, Transport, Application) and maps to real-world protocols.
- Data encapsulation adds headers at each layer: Data → Segment → Packet → Frame → Bits.
- Each layer provides services to the layer above and uses services from the layer below.
- Know which protocols operate at which layer — this is heavily tested on the CCNA.
OSI and TCP/IP Reference Models
Understanding the layered networking models is fundamental to every CCNA topic. Questions about which layer a protocol operates at, or what happens during encapsulation, appear throughout the exam.
The OSI Model (7 Layers)
| Layer | Name | PDU | Function | Examples |
|---|---|---|---|---|
| 7 | Application | Data | User interface to network services | HTTP, FTP, SMTP, DNS, DHCP, Telnet, SSH |
| 6 | Presentation | Data | Data formatting, encryption, compression | SSL/TLS, JPEG, ASCII, MPEG |
| 5 | Session | Data | Establish, manage, terminate sessions | NetBIOS, RPC, PPTP |
| 4 | Transport | Segment (TCP) / Datagram (UDP) | End-to-end delivery, flow control, error recovery | TCP, UDP |
| 3 | Network | Packet | Logical addressing, routing, path determination | IP, ICMP, OSPF, EIGRP, ARP |
| 2 | Data Link | Frame | Physical addressing, error detection, media access | Ethernet, 802.11, PPP, HDLC, STP |
| 1 | Physical | Bits | Physical transmission of raw bits | Cables, connectors, voltages, light signals, hubs |
Memory trick: "All People Seem To Need Data Processing" (Layer 7 down to Layer 1). Or from the bottom: "Please Do Not Throw Sausage Pizza Away."
The TCP/IP Model (4 Layers)
| TCP/IP Layer | OSI Equivalent | Protocols |
|---|---|---|
| Application | Layers 5, 6, 7 | HTTP, FTP, SMTP, DNS, SSH, DHCP, SNMP |
| Transport | Layer 4 | TCP, UDP |
| Internet | Layer 3 | IPv4, IPv6, ICMP, OSPF, EIGRP, BGP, ARP |
| Network Interface | Layers 1, 2 | Ethernet, Wi-Fi, PPP, Frame Relay |
Data Encapsulation
As data moves DOWN the layers from application to physical, each layer adds its own header (and sometimes a trailer):
| Layer | Adds | PDU Name |
|---|---|---|
| Application (7-5) | Application data | Data |
| Transport (4) | TCP/UDP header (source/dest port) | Segment (TCP) or Datagram (UDP) |
| Network (3) | IP header (source/dest IP address) | Packet |
| Data Link (2) | Frame header (source/dest MAC) + FCS trailer | Frame |
| Physical (1) | Converts to electrical/optical signals | Bits |
De-encapsulation is the reverse process when data moves UP the layers at the receiving end.
Key Protocol-to-Layer Mapping
| Protocol | OSI Layer | PDU |
|---|---|---|
| HTTP/HTTPS | 7 (Application) | Data |
| DNS | 7 (Application) | Data |
| DHCP | 7 (Application) | Data |
| SNMP | 7 (Application) | Data |
| SSH/Telnet | 7 (Application) | Data |
| TCP | 4 (Transport) | Segment |
| UDP | 4 (Transport) | Datagram |
| IP (IPv4/IPv6) | 3 (Network) | Packet |
| ICMP | 3 (Network) | Packet |
| OSPF | 3 (Network) | Packet |
| ARP | 2/3 (operates between) | Frame |
| Ethernet | 2 (Data Link) | Frame |
| 802.11 (Wi-Fi) | 2 (Data Link) | Frame |
| STP | 2 (Data Link) | Frame |
On the Exam: ARP is a tricky one — it operates at the boundary of Layers 2 and 3. It resolves Layer 3 addresses (IP) to Layer 2 addresses (MAC). Questions may list it at either layer.
At which OSI layer does a router operate?
What is the correct order of data encapsulation from top to bottom?
Which PDU (Protocol Data Unit) is associated with the Transport layer (Layer 4)?
DNS operates at which layer of the OSI model?