See it once. Then drill it until it's automatic.
The OSI model shows up across the whole N10-009 exam. FlashGenius lets you practice OSI layers, PDUs, protocols, ports, and troubleshooting with domain drills, mixed practice, exam simulation, flashcards, and Smart Review.
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What is encapsulation in the OSI model?
Encapsulation is the process of wrapping data with protocol information as it moves down the OSI model on the sending device. Each layer adds its own header (and, at the Data Link layer, a trailer) so that the matching layer on the receiving device knows how to handle it. The receiving device then performs decapsulation: as the data moves up its stack, each layer reads and removes the header its peer added, until the original message reaches the application.
A useful way to think about it: the Transport layer's header is addressed to the receiver's Transport layer, the Network layer's header to the receiver's Network layer, and so on. This is sometimes called "peer-layer communication." Your data never actually jumps sideways between layers — it goes all the way down, across the medium as bits, and all the way back up.
The PDU name changes at each layer
The exam expects you to know the Protocol Data Unit (PDU) name at each stage. From the upper layers down: the unit is called Data at Layers 7–5, a Segment (or Datagram for UDP) at Layer 4 once the Transport header is added, a Packet at Layer 3 once the IP header is added, a Frame at Layer 2 once the MAC header and FCS trailer are added, and finally Bits at Layer 1 when it is transmitted as signals.
OSI Layer Reference for Network+ N10-009
Use this as a quick reference. Connect each layer to its PDU, its core job, the kind of address or information it adds, and the devices that operate there.
| Layer |
PDU |
Core job |
Adds / uses |
Example protocols |
Devices |
| 7 Application | Data | Interface to user apps and network services | App requests | HTTP, HTTPS, DNS, SMTP | — |
| 6 Presentation | Data | Format, encrypt, compress | Encoding, TLS | TLS/SSL, JPEG, ASCII | — |
| 5 Session | Data | Establish, manage, end sessions | Session control | RPC, NetBIOS | — |
| 4 Transport | Segment | Reliability, segmentation, ports | TCP/UDP header, port numbers | TCP, UDP | — |
| 3 Network | Packet | Logical addressing and routing | IP header, source/dest IP | IP, ICMP, OSPF | Router, L3 switch |
| 2 Data Link | Frame | Physical addressing on the local link | MAC header + FCS trailer | Ethernet, ARP, switching | Switch, NIC, bridge |
| 1 Physical | Bits | Transmit raw signals on the medium | Electrical / optical / radio signals | Cabling, signaling | Hub, cable, transceiver |
Remember the layer order
Two classic mnemonics. Use the top-down one when you reason about encapsulation (Layer 7 → 1), and the bottom-up one when you reason about decapsulation or build the model from the wire up (Layer 1 → 7).
Top-down (Layer 7 → 1)
All People Seem To Need Data Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical
Bottom-up (Layer 1 → 7)
Please Do Not Throw Sausage Pizza Away
Physical · Data Link · Network · Transport · Session · Presentation · Application
Now you try: quick recall check
Watching is not learning — answer five questions to lock it in. Each answer explains the why.
Keep going with Network+ practice
Encapsulation is foundational, but N10-009 also tests networking concepts, implementation, operations, security, and troubleshooting. Layer the rest of the exam on top of what you just learned.
OSI Encapsulation FAQ
What is the difference between encapsulation and decapsulation?
Encapsulation happens on the sending device as data moves down the OSI stack (Layer 7 to Layer 1), with each layer adding its header. Decapsulation happens on the receiving device as data moves up the stack (Layer 1 to Layer 7), with each layer removing the header its peer added.
What are the PDU names at each OSI layer?
Layers 7–5: Data. Layer 4: Segment (TCP) or Datagram (UDP). Layer 3: Packet. Layer 2: Frame. Layer 1: Bits.
Which layer adds a trailer as well as a header?
The Data Link layer (Layer 2). It adds a frame header containing MAC addresses and a trailer called the Frame Check Sequence (FCS) for error detection.
Which layer adds IP addresses?
The Network layer (Layer 3) adds the IP header with source and destination IP addresses for logical, end-to-end routing.
Is this the same as the TCP/IP model?
They are related but not identical. The TCP/IP model collapses the seven OSI layers into four (or five). Network+ expects you to know the seven-layer OSI model and how data is encapsulated through it.
About this explainer
This interactive OSI encapsulation walkthrough was created by FlashGenius to help CompTIA Network+ candidates visualize how data is encapsulated and decapsulated through the seven OSI layers for the N10-009 exam. Last updated: June 2026.
