OSI & TCP/IP Study Guide
Why models exist
The OSI and TCP/IP models divide networking into layers so each layer has a clear responsibility.
Each layer provides services to the layer above and uses services from the layer below.
OSI Layers
7. Application: Network services for applications (HTTP, DNS, SMTP, SSH).
6. Presentation: Data representation, encryption, compression (TLS, encoding).
5. Session: Establishes, manages, and terminates communication sessions.
4. Transport: End-to-end delivery, segmentation, reliability, flow control, ports (TCP/UDP).
3. Network: Logical addressing and routing using IP. Routers operate here.
2. Data Link: Framing, MAC addressing, FCS error detection, switching. Switches operate here.
1. Physical: Transmits bits as electrical, optical, or radio signals.
TCP/IP Mapping
Application = OSI Layers 5–7
Transport = OSI Layer 4
Internet = OSI Layer 3
Network Access = OSI Layers 1–2
Encapsulation
Application data becomes a TCP segment, then an IP packet, then an Ethernet frame, then bits on
the wire. At the receiver, decapsulation occurs in reverse.
Golden CCNA Facts
• MAC addresses are hop-by-hop and change at every router.
• IP addresses are end-to-end (except technologies like NAT).
• Ethernet FCS detects corruption on one link only.
• TCP provides end-to-end reliability using sequence numbers, acknowledgments, retransmissions,
and checksums.
• The Ethernet preamble synchronizes the receiver's clock; the SFD marks the start of the frame.
• A switch forwards frames using MAC addresses; a router forwards packets using IP addresses.
• Logical peer communication (e.g., TCP↔TCP) always physically travels down the sender's stack,
across the network, and up the receiver's stack.
PDU Names
Application: Data • Transport: Segment (TCP)/Datagram (UDP) • Network: Packet • Data Link:
Frame • Physical: Bits