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Confused regarding routers/modems

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by aph002 1 view

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I recently completed a networking course, and up until now I have been confident in my knowledge.

Throughout this course, however, modems were not once mentioned, and I had no idea they existed. I was led to believe that routers handled all communication with the internet, so that the structure of a simple network would be:

Internet --> Router --> Switch --> Hosts

Doing my own research, it seems as though modems are in fact the devices that communicate with the internet, whereas routers simply route within a LAN, which leaves me extremely confused as I was under the impression that was the job of the switch.

So my question is, what purposes do routers and modems serve in a network?

Thank you.

4 replies

by rproffitt

Your view could be true. But for most you'll find a modem or more likely a combination box that has the modem plus router, switch and the WiFi Access Point all in one box.

Long ago each of these were in their own box but today, all in one for most homes.

by kees_b

The modem is the connection between the LAN and the outside world. Thirty years ago they connected to a phone-line and they connected with awful audible beeps to another modem at the ISP to give you a 300 bits/second connection. That modem translated the beeps to bits, like a glass fiber modem translates light to bits.

The router connects the LAN to the modem, and via the modem to the outside world. You don't need a router to connect one PC only to a modem via an Ethernet-cable. Like you connected your 300 baud modem directly to a serial port on your PC.

by aph002

That makes a lot of sense, so the purpose of a router is communication with other devices on a LAN.

I understand the need for a router, say, if within one LAN I have 2 subnets and I want to communicate between the routers (and subsequently the devices) of each subnet.

But say I have a network such as:

Internet --> Modem --> Router --> Switch --> 3 Hosts

Why do I require a router in this situation? Wouldn't just a switch work just as well? e.g.

Internet --> Modem --> Switch --> 3 Hosts

by rproffitt

Let's tackle the NO ROUTER setup first. Today's generic home router is so your ISP can hand you an Internet IP and the router handles the work necessary to share this single IP.

BUT if you want to omit the router you work with your ISP to arrange for more IP addresses. NO ONE DOES THIS!!! Why? Because the ISPs today charge for each address. Let's say your ISP costs are 50 USD a month. Want a second IP address? ISPs that do that will charge 50 USD for each additional IP address. A router is far less costly.

-> Second topic. SUBNETS. This can be done but as folk quickly figure out with today's switched networks, there is NO SPEED GAIN and barely any added security. But hey, if you want to learn more, set it up and learn why no one does this in a home or small business.

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