Introduction While studying for CompTIA Network+, I couldn't totally grasp the PDU names in the OSI model. I could memorize them, but I never understood why the names are what they are. One day, while ordering some items on Amazon, it hit me — the ordering process maps perfectly to the OSI model. So I decided to write it down. The OSI model is a framework that divides network communication into 7 layers. Separating responsibilities by layer makes it easier to identify which layer is causing a problem. Layer Name 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Note: This mnemonic goes from Layer 1 to 7 (bottom to top). PDU (Protocol Data Unit) is the name given to data at each layer. Each time data passes through a layer, it gets a new name. Layer PDU name 7–5 Data 4 Segment 3 Packet 2 Frame 1 Bit We can memorize them: Data → Segment → Packet → Frame → Bit -Encapsulation: The process of adding headers to data as it moves down the layers (sender side). We can memorize them: -En = put in an envelope → Top to Bottom (7-1), adding header when sending. This section is the main part of this article. Data represents the items we ordered in Amazon. It can be difficult to deliver them, if the order consists of multiple items. Without numbers, it would be impossible to identify which piece is missing if they arrive out of order. In this process, divided items are put into boxes and labeling each of them. The labels are useful to determine routes to deliver. The destination address (IP address) always stays the same, but the package doesn't travel directly to the final destination -it goes through multiple delivery centers along the way. The actual delivering process is going through many delivering center in each locations until completing delivering, and relay labels, shows destination center, are replaced each time. -Destination address label (IP Address) = Final destination = stays same Actually, those boxes are transported physically by track. [Sending side: Encapsulation] [Receiving side: Decapsulation] Bits ← Network ↓ Removing relay labels at each delivery center Frames → Packets → Segments ↓ Reassembling and unpacking Items (Data)