What does OSI stand for?
OSI stands for Open Systems Interconnection. A seven-layer reference model for how network communication is divided: Physical, Data Link, Network, Transport, Session, Presentation, Application. Exams map devices and protocols to layers: switches at 2, routers at 3, TCP and UDP at 4, HTTP at 7.
The OSI model divides network communication into seven layers so that each job can be described and troubleshot separately. From the bottom: 1 Physical (signals and cables), 2 Data Link (frames and MAC addresses), 3 Network (packets, IP addresses and routing), 4 Transport (TCP and UDP, ports), 5 Session (opening and closing dialogues), 6 Presentation (format and encryption of data) and 7 Application (the protocols programs use, such as HTTP). It is a reference model, not a set of protocols you install.
OSI in practice: an example
A user cannot open a website. The technician works up the layers: the link light is on (layer 1), the switch has learned the MAC address (layer 2), a ping to the gateway succeeds (layer 3), but a connection to port 443 times out (layer 4). The fault is a firewall rule, not the cable.
What OSI is confused with
OSI vs TCP/IP model: what is the difference?
The TCP/IP model describes the same communication with four layers: link, internet, transport and application. Its application layer covers OSI layers 5 to 7. Real networks run TCP/IP protocols; OSI is the vocabulary used to describe them.
Which exams cover OSI?
OSI is on the official CompTIA acronym list for one of the four exams covered here.
- Network+ N10-009: 1.0 (Networking Concepts); 5.0 (Network Troubleshooting).
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. At which OSI layer does a router make its forwarding decision?
- Layer 1, Physical
- Layer 2, Data Link
- Layer 3, Network
- Layer 7, Application
Show the answer
C. Layer 3, Network. Routers forward packets by IP address, which belongs to the Network layer. Switches work with MAC addresses at layer 2.
2. A cable has a broken wire and the link light stays off. Which OSI layer has the fault?
- Physical
- Transport
- Session
- Presentation
Show the answer
A. Physical. Cables, connectors and signals are the Physical layer. Nothing above it can work until the link is up.
Sources: ITU-T Recommendation X.200: Open Systems Interconnection, Basic Reference Model; Microsoft Learn: Windows network architecture and the OSI model; RFC 1122: Requirements for Internet Hosts, Communication Layers. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.