Routers and Modems
When Kite Studio's internet went down last month, the office manager restarted "the router". The box she restarted was actually the ISP's fibre terminal, a kind of modem, and the real router sat one shelf above it. At home one plastic box often does both jobs, which is why the words get blurred. In an office they are separate, and knowing which is which saves hours of troubleshooting.
The rule
A switch joins devices; a router joins networks; a modem joins the line.
Router — connects different IP networks and moves packets between them. It reads the destination IP address of each packet and checks its routing table, a list of known networks and where to send traffic for each. Kite Studio's router has two sides: its LAN port, with the address 192.168.10.1, and its internet port, with the public address 203.0.113.10.
| Destination network | Send to |
|---|---|
192.168.10.0/24 (the office) |
directly out of the LAN port |
0.0.0.0/0 (everything else) |
the ISP, next hop 203.0.113.1 |
The last line is the default route: "if nothing more specific matches, send it here". From the laptops' point of view, the router's LAN address 192.168.10.1 is their default gateway, the door they use for anything off the local network. Routers also stop broadcasts, so an ARP shout on the office LAN never escapes to the internet.
Modem — short for modulator-demodulator. It converts the computer world's digital frames into the kind of signal the provider's line carries, and back: electrical tones on a phone line (DSL), signals on a coaxial TV cable, or light on fibre (where the box is usually called an ONT, optical network terminal). A modem does not choose paths; it just makes the line usable.
The ISP's home "router" is typically a bundle: modem + router + switch ports + Wi-Fi access point + DHCP + NAT in one case.
The shortcut
Trace outward from a laptop and ask at each box: "Is it joining devices, networks or a line?" Devices on one LAN → switch. Two different IP networks → router. A provider's cable or fibre → modem.
Laptop to switch
Same LAN, frames forwarded by MAC: the switch joins devices.
The trap is expecting a router to forward traffic between two laptops on the same LAN. They talk through the switch; the router is involved only when a packet must leave the network.
Drills
Answer fast — trust the rule.
What does a router read to decide where a packet goes?
In Kite Studio's routing table, what does 0.0.0.0/0 mean?
Which box converts the office's frames into light on the provider's fibre?
Asha copies a file from 192.168.10.20 to her laptop at 192.168.10.57. Does the traffic pass through the router?
Remember
- A switch joins devices; a router joins networks; a modem joins the line.
- A router forwards by destination IP using its routing table;
0.0.0.0/0is the default route. - The laptops' default gateway is the router's LAN address,
192.168.10.1.