The TCP/IP Stack and Protocols
The layered network model
Network communication is organised into layers, each with a specific job. A layer only interacts with the layers directly above and below it, so any layer can be changed without affecting the others (abstraction and easier development). The TCP/IP model has four layers.
The four layers
| Layer | Job | Example protocols |
|---|---|---|
| Application | Provides network services to programs; formats data | HTTP(S), FTP, SMTP, IMAP, POP3, DNS |
| Transport | Splits data into packets/segments, adds port numbers, manages reliability and reassembly | TCP, UDP |
| Internet (Network) | Adds IP addresses; routes packets across networks | IP |
| Link (Network access) | Physical transmission over the hardware; adds MAC addresses | Ethernet, Wi-Fi |
Data passes down the layers at the sender (each adds its own header — encapsulation) and back up at the receiver (each strips its header).
TCP vs UDP
- TCP (Transmission Control Protocol): connection-oriented and reliable — numbers packets, checks for errors, resends lost packets, reassembles in order. Used for web, email, file transfer.
- UDP (User Datagram Protocol): connectionless and faster but unreliable — no guarantee of delivery or order. Used for streaming, gaming, VoIP where speed matters more than perfection.
Ports and sockets
- A port number identifies which application/service a packet is for (e.g. HTTP = 80, HTTPS = 443, FTP = 21, SMTP = 25).
- A socket = an IP address + a port number, uniquely identifying an endpoint of a connection.
IP addresses and DNS
- An IP address identifies a device on a network (IPv4 = 32-bit, e.g. 192.168.0.1; IPv6 = 128-bit for far more addresses).
- DNS (Domain Name System) translates human-readable domain names (e.g. example.com) into IP addresses via a hierarchy of DNS servers.
- A MAC address is a unique physical hardware address of a network card (used at the link layer).
Worked example
Which layer adds the IP address, and which adds port numbers?
- The Internet layer adds IP addresses (for routing between networks); the Transport layer (TCP) adds port numbers (to reach the right application). ✓
Common mistakes
- Confusing IP addresses (Internet layer, logical) with MAC addresses (Link layer, physical).
- Saying UDP is reliable — TCP is reliable; UDP trades reliability for speed.
- Mixing up which protocols sit at which layer.
Exam tips
- Learn the four layers in order with their jobs and example protocols.
- Contrast TCP (reliable, ordered) vs UDP (fast, connectionless) with a use for each.
- Know what a socket, port, IP address, MAC address and DNS each do.
Key facts to remember
- TCP/IP layers: Application, Transport, Internet, Link — each adds/strips a header (encapsulation).
- TCP = reliable/ordered; UDP = fast/connectionless; ports identify the application, sockets = IP + port.
- IP address = logical device address; MAC = physical; DNS maps domain names → IP addresses.