Today, ISPs give you a "Gateway" or "Combo Box" that does everything. This creates confusion. To troubleshoot network issues, you must separate the roles of the Modem (Analogue/Digital Translator) and the Router (Traffic Cop).
The Modem
Role: Modulation / Demodulation.
Input: Analog signal from the wall (Coaxial Cable, Phone Line, Fiber Light).
Output: Digital Ethernet signal.
It speaks the language of the ISP infrastructure. Without it, you have no internet access.
The Router
Role: Routing packets between devices.
Input: One Public IP (from Modem).
Output: Many Private IPs (to your Phone, Laptop, TV).
It assigns local IPs via DHCP and manages NAT so multiple devices can share one connection.
1. Why separate them?
Power users often buy their own Router (e.g., a gaming router or Mesh WiFI) and put the ISP box into "Bridge Mode".
Bridge Mode turns off the Router part of the ISP box, making it just a dumb Modem.
This avoids "Double NAT" issues (which break online gaming) and gives you better WiFi coverage.
2. What about the Switch?
Most routers have 4 Ethernet ports on the back. That is technically a built-in Switch.
A Router connects networks (Your LAN to the Internet).
A Switch connects devices (Your PC to your Printer).