Networking stacks are layered so each level solves one problem and talks to the level above and below through well-defined interfaces. The OSI model (7 layers) is a reference framework; the TCP/IP stack (4 layers) is what the Internet actually runs. Understanding both lets you read captures, configs, and RFCs without getting lost.

OSI — seven logical layers
Click each layer — what it defines
Press Next → to walk from the application layer down to the wire and back
OSI ENCAPSULATION · SENDER ↓
↓ Sender
L7 Application
Data
L6 Presentation
L5 Session
L4 Transport
L3 Network
L2 Data Link
L1 Physical
Data
GET /index.html HTTP/1.1
↓
L7 Application
Protocol:HTTP / HTTPS / DNS
+ Adds:
HTTP Header
Unit: Data
User application generates the message. HTTP adds method, URL, and headers — what developers write and read.
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L7 adds HTTP Header → unit becomes: Data
TCP/IP — four practical layers
Hover rings from outside in — how packets are scoped
Network AccessInternetTransportApplication
Hover rings from outside in — how packets are scoped
How OSI maps to TCP/IP
ConcernOSI layersTCP/IP layer
End-user data & APIs7 ApplicationApplication (incl. 5–7)
Reliable / unreliable delivery4 TransportTransport
Global paths & addresses3 NetworkInternet
Local link & bits2 + 1 Data Link + PhysicalNetwork Access
Encapsulation on the wire
Click through — PDU names at each step
Data
App payload
Segment/DG
+TCP/UDP hdr
Packet
+IP header
Frame
+L2 header/trailer
Bits
Physical
▶
Click the center — conceptual peers
Same host stack
🌍Application
🔀Transport
🛰️Internet
📡Link
▶

Takeaways

Neither OSI nor TCP/IP cares about vendor hardware — they are abstract models. Troubleshooting follows the layers: physical link up? IP address and route? TCP connection? TLS and HTTP? Peeling the stack from bottom or top until you find the broken layer is the core skill.
OSI = reference, 7 layers
TCP/IP = implementation, 4 layers
Encapsulation hides details
Peer layers talk logically
Names: frame / packet / segment
RFC 1122 and modern texts sometimes group layers differently — focus on function (who adds which header) not memorizing numbers alone.