What is the difference between a DSL modem and a router?

By Lisa Phifer

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Those little beige boxes might all look alike, but DSL modems and routers aren't identical. Each has an important role to play, so be sure to buy the box you really need for your home network. This article explains what modems and routers do, how they work together in networks, and how to decide whether to buy one or both for your home.

Accessing the Information Highway

Picture the Internet as the system of highways and side-streets that surround your home. When you retrieve a Web page or download an email message, that traffic flows across the Internet. To reach your home, it must exit the “information highway” (your Internet service provider) onto your side street (your DSL access link).

Now think of your DSL modem as your Internet front door – the portal through which those bits of information actually enter your home. But once that traffic passes through your front door, where does it go? Picture your DSL router as a traffic cop that directs that information to the laptop in your den or desktop upstairs. To receive a Web page or mail message, information must first enter your home (your modem’s job) and then be directed to the right PC (your router’s job).

Without the modem, the router has nothing to do. But the modem can deliver traffic to a single destination, with or without a router. That's probably how you accessed the Internet before broadband, using dial-up. Your desktop had an analog modem that it used to call your Internet Service provider, sending information over the telephone lines. Well, today, you could do the same thing with broadband by connecting a DSL modem directly to a PC, usually with a USB cable. And, if you have just one computer in your home, that’s all you really need to access the Internet (see Figure 1).
 

Figure 1. Accessing the Internet from a single computer with a DSL modem

Building a home network

But, if you have more than one PC in your home, you probably want to let all of those computers share one broadband Internet connection. To do that, you must add a broadband router. The router looks at all of the traffic delivered by your DSL (or cable or satellite) modem. Based on the destination Internet Protocol address of each message, the router relays traffic to the appropriate device inside your network (see figure 2).
 

Figure 2. Using a shared DSL modem and router to create a home network

Any router can direct traffic that enters and exits your network. But many broadband routers provide additional services that are handy for home networks.

Most broadband routers have built-in Internet firewalls. A firewall can stop unwanted traffic from entering (and sometimes leaving) your network, much as a doorman controls who is allowed to enter or leave a building through a guarded door. By default, most broadband routers block all unsolicited inbound traffic, while letting everyone inside your home network have unfettered outbound Internet access. Just about every home network needs an Internet firewall, so don’t buy a router without one.

Many broadband routers have a small integrated Ethernet switch. Typically, this switch has four Local Area Network (LAN) ports that can be used to connect a few PCs and printers to the router. Most home users have at least one device they’d like to cable directly to their router. Even if you don’t, those ports will come in handy during set-up.

Some broadband routers have an integrated Wireless Access Point (AP). This feature is handy to connect laptops and PCs throughout your home, without stringing Ethernet cables across floors and through walls. If your router doesn’t have an integrated AP, don’t worry – you can add an AP later by plugging one into a router switch port.

There are many other features found in broadband routers – from built-in print servers to Web filters and network anti-virus. Don’t assume that all broadband routers are alike. If your needs are basic, an inexpensive router purchased from your local office supply store will probably do the trick. If you plan to use Internet phone or TV service or play on-line games or enable wireless throughout your home, take a closer look before you buy.

All together now

Unlike broadband routers, modems are intimately tied to your Internet Service. If you have DSL service, you’ll need a DSL modem that’s compatible with your provider’s specific flavor of DSL. (If you have cable or satellite broadband, you’ll need a modem compatible with those networks.) As a result, most consumers purchase digital modems with their broadband service. If an installer visits your home, he or she will probably test your new Internet service by connecting that modem to a single PC.

As home networks grew popular, many providers started to sell routers along with broadband services. In this case, the installer brings both a modem and a router and tests the combination to make sure they work together during set-up. You aren’t usually required to buy a broadband router from your provider – and if you have special needs, you may choose not to. But if you need both a modem and a router, buying the combo can reduce headaches if and when you have an outage and you need to call tech support.

 
Figure 3.   

In fact, some broadband routers go beyond the call of duty by combining both modem and routing functionality within a single box (Figure 3). This is handy for your provider – now they have just one box to install and support. But a combined broadband modem/router can also be handy for you as a consumer. Why?

• You’ll have just one box to plug into your surge protector.
• If Internet service goes down, you have just one box to restart.
• If things stop working, there will be fewer possible problems.
• If you buy Internet phone or TV service, router compatibility is virtually guaranteed.

However, a combined modem/router limits your options. For example, you may not want to put a wireless router in the location where DSL enters your home. Some employers supply pre-configured broadband routers to workers to ensure security and promote VPN compatibility. A combined modem/router may also end up costing more up-front or per-month, compared with just buying your own broadband router.

The bottom line: consider the big picture before accepting that “free with 2-year commitment” modem/router from your broadband provider. If you know the difference between a broadband modem and a router, and give those router features some real thought when you purchase broadband Internet service, you won’t get stuck with a router you don’t want or need.