TCP/IP model
Also known as: internet protocol suite, TCP/IP stack
The TCP/IP model is the four-layer framework that describes how data moves across the internet, consisting of the application, transport, internet, and network access layers. It is the practical model real networks are built on.
The TCP/IP model organizes networking into four layers, each handling one part of the job and passing data to the layer below. The application layer covers protocols users interact with, such as HTTP, DNS, DHCP, SMTP, and FTP. The transport layer provides end-to-end delivery between processes through TCP or UDP. The internet layer handles logical addressing and routing between networks using IP, along with ICMP. The network access layer (sometimes split into data link and physical) covers Ethernet, wireless, MAC addressing, ARP, and the actual transmission of bits on a medium.
Data travels down the stack through encapsulation. The application hands data to the transport layer, which adds a header and creates a segment. The internet layer adds an IP header with source and destination addresses, producing a packet. The network access layer adds a frame header and trailer with MAC addresses, producing a frame that is transmitted as bits. On the receiving host the process reverses — de-encapsulation — with each layer stripping its own header and passing the payload upward.
The seven-layer OSI model maps onto this closely. OSI's application, presentation, and session layers correspond to the TCP/IP application layer; OSI transport and network map one-to-one onto transport and internet; and OSI data link and physical together form the network access layer. Networking professionals still use OSI layer numbers as shorthand — a "Layer 2 switch" forwards by MAC address, a "Layer 3 device" routes by IP address — even though the TCP/IP model describes how the protocols are actually implemented.
On the Cisco CCNA exam, the TCP/IP model is foundational. You are expected to identify which layer a protocol or device belongs to, describe encapsulation and the resulting protocol data units, map TCP/IP layers to OSI layers, and apply the model when troubleshooting connectivity layer by layer.
Key takeaways
- The TCP/IP model has four layers: application, transport, internet, and network access.
- TCP and UDP operate at the transport layer; IP addressing and routing happen at the internet layer.
- Encapsulation adds headers going down the stack, producing segments, packets, and frames.
- OSI's seven layers map onto TCP/IP's four, and OSI layer numbers remain common shorthand.
- The CCNA exam tests layer identification, encapsulation, and layered troubleshooting.