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A "modem" is a very common piece of equipment and there is one in every Internet-connected home, though you may not always be aware that it is there.

Simply put a modem is a device that "translates" the Internet signal from the type of signal your Internet Service Provider (abbreviated ISP) uses to the type of signal your equipment uses.

When you get a signal from your ISP you have exactly one "address" on the Internet.  The modem is the piece of equipment which gives you that address.  All Internet traffic going to your location, regardless how many connected devices you have, go to that single Internet address.  If you have more than one connected device you also need a "router", explained in the Router section.  Many modern modems also have a router built in.

Generally the modem is pretty easy to find.  It will be a piece of equipment connected directly to the incoming line from your ISP.  If you are using DSL (Digital Service Line), that's a phone line.  If you're using cable Internet, it's a coaxial cable and so on.  However, some services do not use a modem inside the house.  Instead there is a box outside the house which has the modem in it and the line coming into the house is already the proper signal for your equipment.

One of the most common issues when you have Internet connectivity issues is that the modem needs to be "rebooted".  This simply means that it needs to be turned off and then back on.  Some have a power button on them, but many do not.  In that case you need to unplug the power cable from your modem, wait 5 seconds and then plug it back in.  It will take several minutes for the modem to "reboot" and you will not have Internet until it is done with all of its checks and connecting.  This usually takes about 2 or 3 minutes.  The power cable for a modem is always a cord from a DC converter box and is always black, as far as I know.  It will be a black cord which is round, but not nearly as thick as a coaxial cable and, as far as I have seen, will always be on one end of the modem.  If it's in the middle of the cables it's not the power cable.  If you are unsure trace the cable back and see where it goes.  If it plugs into the power outlet, that's the power cable.  You can unplug it either from the wall or directly from the modem, which is usually easier.

There are a couple of points of confusion for DSL modems which sometimes trip people up.  DSL comes over the phone line, but it's nothing like dialup.  With DSL you can be on the Internet and the phone at the same time.  The way this works is that it uses higher frequencies than you use for voice calls.  With simple filters in place these frequencies can be filtered out so that you don't hear them.

The first point of confusion is connecting the phone line.  Most DSL modems have two connectors in the back where a phone line will fit.  One is usually labeled "Line" while the other is usually labeled "Phone".  The one labeled "Line" is for connecting the incoming line from the wall.  The one labeled "Phone" is for connecting a telephone.  Remember how I said you can be on the Internet and telephone at the same time?  That's why this port exists.  Since the DSL modem needs to connect to your phone line they usually add a phone port in the back so that you do not have to run a second line.

The second point of confusion is the filters.  The filters are for filtering the high-pitched "noise" that is the DSL modem's way of "talking" out of your phone lines, so these filters go in line with your phones, not with the DSL modem.  If you put one in the line on your DSL modem it will not be able to get a signal.  If your phone is connected to the "Phone" port on the back of the DSL modem you do not need a filter.  There is one built into the modem for that port.