You've learned network architecture foundations.
Models: OSI has layers for teaching. TCP/IP has for real protocols.
OSI Layers: Physical transmits bits. Data Link delivers frames. Network routes packets. Transport adds reliability. Session, Presentation, Application handle higher functions.
PDUs: Data → Segment → Packet → Frame → Bits. Encapsulation adds headers down. Decapsulation removes them up.
MTU: Limits packet size per link. PMTUD discovers path minimum. Fragmentation splits; reassembly joins.
You're ready for advanced network design.