Network devices map onto the TCP/IP model according to the highest layer whose information they process. Switches and access points mainly operate at the network access layer, routers at the internet layer, while hosts and some firewalls operate across multiple layers.
The Mapping Mechanism
The four-layer TCP/IP model explains how data is prepared, addressed, transmitted, and received. As data moves down the model, each layer adds control information through encapsulation; the receiver removes it through decapsulation.
| TCP/IP layer | Main function | Typical device or system |
|---|---|---|
| Application | Network services using HTTP, DNS, or SMTP | Hosts, proxy servers, application gateways |
| Transport | End-to-end communication using TCP or UDP port numbers | Hosts, transport-layer firewalls, load balancers |
| Internet | Packet routing between networks using IP addresses | Routers, layer 3 switches, packet-filtering firewalls |
| Network access | Local transmission of frames and bits using MAC addresses | Switches, wireless access points, network interface cards, bridges, hubs |
A switch examines MAC addresses and forwards frames within a local network. A router examines IP addresses and selects a route between networks. A wireless access point bridges wireless devices to a wired LAN, placing its main function at network access.
Some devices combine functions. A basic firewall may filter IP addresses at the internet layer and port numbers at transport, while an application firewall inspects application data. A home “router” may contain a router, switch, access point, modem, and firewall.
A common misconception is that every physical device belongs exclusively to one layer. Classification depends on the function being performed and the protocol information examined, not the device’s casing or marketing name.
Exam Technique
For an IB A2.1 response, name the layer and justify it with the relevant identifier: MAC address for network access, IP address for internet, port number for transport, or application protocol for application. Avoid replacing the four-layer TCP/IP model with the seven-layer OSI model.
