Follow a Request Through the Layers

About this interactive

The OSI model groups what protocols do into seven layers. Data is wrapped from the top down on the sending side and unwrapped from the bottom up on the receiving side. On the way down: the application layers (5 to 7) hand over data; layer 4 adds a TCP header with the destination port, making a segment; layer 3 adds an IP header with the destination IP address, making a packet; layers 2 and 1 wrap it in a frame and send it as bits, starting with a preamble that lets the receiver synchronise to the signal. Layers 1 and 2 only deliver to the next device. Each router strips them off, reads the IP header to choose the next hop, and wraps the packet in whatever the next link uses: Ethernet on a LAN, PPP or HDLC between routers. The IP addresses and the TCP segment travel end to end. On the way up, the receiver removes the frame, checks the IP address, reads the port in the TCP header, and hands the data to the program on that port. Because each layer has its own job, one layer can be upgraded, such as a new kind of link, without changing the others.

What you'll learn

Trace one web request through the OSI layers: which header each layer adds on the way down, what the unit is called at each layer, and the order the headers come off on the way up
Predict what each router does at a hop: strip the layer 1 and 2 wrapping, read the IP header, and wrap the packet in whatever layer 1 and 2 technology the next link uses, such as Ethernet, PPP or HDLC
Explain why the OSI model lets one layer be upgraded without changing the layers above it

Aligned to

CompTIA Network+

1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.

2.1 Explain characteristics of routing technologies.

Cisco CCNA

1.1 Explain the role and function of network components

Cisco CCST Networking

1.1 Identify the fundamental conceptual building blocks of networks

4.4 Explain basic routing concepts

ISC2 CC

4.1 Understand network security

Key terms

Encapsulation: The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
De-encapsulation: The process of removing protocol headers from data as it travels up the OSI model layers on the receiving device.
Open Systems Interconnection Model: A conceptual framework that standardizes network communication into seven distinct layers.
Protocol Data Unit: The unit of information passed between peer entities at the same OSI layer, with layer-specific names: segment or datagram at the Transport layer, packet at the Network layer, frame at the Data Link layer, and bit at the Physical layer. Understanding PDU terminology is fundamental to troubleshooting network communication at each layer.
Point-to-Point Protocol: A data link protocol used to establish a direct connection between two nodes.
Preamble: A 7-octet sequence of alternating 1s and 0s transmitted at the start of an Ethernet packet to synchronize the clock timing between communicating devices.
Router: A network device that forwards data packets between networks based on IP addresses.

Topics

Encapsulation De Encapsulation Open Systems Interconnection Model Protocol Data Unit Point To Point Protocol Interactive Predict

About TechKnowSurge

TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.

Explore free tools and programs →