1.1 OSI and TCP/IP Architectural Layers & Encapsulation

🎯 Key CompTIA Network+ Exam Takeaways

• Data encapsulates downward through the stack: Layer 4 Segment, Layer 3 Packet, Layer 2 Frame, Layer 1 Bits.
• Layer 4 provides transport reliability via TCP three-way handshake (SYN, SYN-ACK, ACK) or connectionless UDP delivery.
• Layer 2 switches make forwarding decisions based on hardware MAC address tables; Layer 3 routers forward packets based on IP routing tables.

The Open Systems Interconnection (OSI) 7-layer reference model provides a standardized architectural framework for understanding network communications. Application, Presentation, and Session layers manage user data, data translation, and connection states.

At the Transport layer (Layer 4), data is divided into Segments when using TCP or Datagrams when using UDP. TCP enforces end-to-end reliability through sequence numbering, acknowledgments, and flow control windows. Layer 3 encapsulates segments with logical IP addresses into Packets.

At Layer 2 (Data Link), packets are framed with physical MAC headers and a Frame Check Sequence (FCS) trailer to detect errors over local links. Finally, Layer 1 converts frames into physical electromagnetic or optical signals transmitted across physical media.

⚠️ Common Pearson VUE Exam Traps

• Do not confuse Layer 4 Segments with Layer 3 Packets or Layer 2 Frames.
• Remember that MAC addresses are stripped and rewritten at every Layer 3 hop, whereas source/destination IP addresses remain intact across transit routers.