Mac OS X Snow Leopard: The Missing Manual (Missing Manuals)

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19.7. Internet Sharing

If you have cable modem or DSL service, you're a very lucky individual. Not only do you benefit from great speed when surfing the Web or processing email, but your connection is on full-time . You never have to wait for some modem to dial (screeching all the way), and wait again for it to disconnect. Too bad only one computer in your household or office can enjoy these luxuries .

Actually, it doesn't have to be that way. You can spread the joy of high-speed Internet to every Mac (and PC) on your network in either of two ways:

  • Buy a router . A router (sometimes called a residential gateway ) is a little box, costing about $50, that connects directly to the cable modem or DSL box. In most cases, it doubles as a hub, providing multiple Internet jacks into which you can plug your Macs and PCs. As a bonus, a router provides excellent security, serving as a firewall to keep out unsolicited visits from hackers on the Internet. (If you use a router, turn off Mac OS X's own firewall, as described on Section 12.11.)

    FREQUENTLY ASKED QUESTION

    Internet Sharing Using a Modem

    My eMac isn't anywhere close to a phone jack. Can I use Internet Sharing to get online via the dial-up modem of my upstairs Power Mac?

    As long as your eMac is connected to the gateway machine via a network (Ethernet or AirPort), the answer is yes.

    Think about it: Dial-up modems and Bluetooth cellphones may be listed in the Network panel of System Preferences, but they're only connections to the Internet , not connections to your home network .

    If you want the gateway Mac to share its dial-up or Bluetooth connection with other machines on your network, therefore, the selected Location (Section 19.5) must list both the modem/Bluetooth connection and either Ethernet, AirPort, or FireWire.

    The bottom line: If the Show pop-up menu in your Network panel shows only Internal Modem or only Bluetooth, then you won't be able to turn on Internet Sharing at all.

  • Use Internet Sharing . Mac OS X's Internet Sharing feature is the software version of a router, in that it distributes a single Internet signal to every computer on the network. But unlike a router, it's free. You just fire it up on the one Mac that's connected directly to the Internet ”the gateway computer. (The Internet Sharing feature is also responsible for Mac OS X's software base station feature for AirPort Macs, as described on Section 19.7.2.)

    But there's a downside: If the gateway Mac is turned off or asleep, the other machines can't get online.

Most people use Internet Sharing to share a broadband connection like a cable modem or DSL. But in fact, Internet Sharing works even if the gateway Mac connects to the Internet via dial-up modem or even a Bluetooth cell phone.

Figure 19-7. Ka-ching! Mac OS X's Internet Sharing button just saved you the cost of a $50 hardware router. In this example, the Mac is connected to a cable modem via Ethernet. But other Macs are connected to it via wireless AirPort and by a FireWire cable. And now they can get online, too, even though they're not directly connected to the cable modem.

The only requirement is that the gateway Mac also has a network connection (Ethernet, AirPort, or FireWire) to the Macs that will share the connection.

19.7.1. Turning On Internet Sharing

To turn on Internet Sharing on the gateway Mac, open the Sharing panel of System Preferences. Click the Internet tab and then click Start, as shown in Figure 19-7. (In most setups, you'll want to turn on Internet Sharing only on the gateway Mac.)

To set up sharing, you have to specify (a) how the gateway Mac is connected to the Internet, and (b) how it's connected to the other Macs on your office network:

  • Share your connection from . Using this pop-up menu, identify how this Mac (the gateway machine) connects to the Internet ”via Built-in Ethernet, Internet Modem, or whatever. If you turn on AirPort, you create the software base station effect described in the next section.

  • To computers using . Turn on the checkboxes to teach your Mac how the other Macs are connected to it ”via what form of networking, in other words.

Note: Which checkboxes appear here depend on which kinds of Internet connections are turned on in the Network panel of System Preferences. If the gateway Mac doesn't have an AirPort card installed, for example, or if AirPort is turned off in the current configuration, the AirPort option doesn't appear.
POWER USERS' CLINIC

Internet Sharing as a Bridge

Ordinarily, only one Mac has Internet Sharing turned on: the one that's connected directly to the Internet.

But sometimes, you might want another Mac "downstream" to have it on, too. That's when you want to bridge two networks.

Consider the setup shown here, where there are really two networks: one that uses AirPort, and another connected to an Ethernet hub.

If you play your cards right, all of these Macs can get online simultaneously , using a single Internet connection.

Set up the gateway Mac so that it's an AirPort base station, exactly as described on these pages.

Start setting up the bridge Mac the way you'd set up the other AirPort Macs ”with AirPort selected as the primary connection method, and "Using DHCP" turned on in the Network panel.

Then visit the bridge Mac's Sharing panel. Turn on Internet Sharing here, too, but this time select "To computers using: Built-in Ethernet."

The bridge Mac is now on both networks. It uses the AirPort connection as a bridge to the gateway Mac and the Internet ”and its Ethernet connection to share that happiness with the wired Macs in its own neighborhood.

Now visit each of the other Macs on the same network. Open the Network panel of System Preferences. Using the Show pop-up menu, choose AirPort, Built-in Ethernet, or FireWire ”whichever reflects how each Mac is connected to your network. Then, from the Configure pop-up menu, choose Using DHCP. Leave everything else blank. Finally, click Apply Now.

As long as the gateway Mac remains turned on, you should find that both it and your other Macs can get onto the Internet simultaneously, all at high speed. (Even Windows PCs on the same network can also get online, as long as you set them up to use DHCP just as you did your "downstream" Macs.)

Note: If you're like most people, your gateway Mac has only one Ethernet port. How, you may wonder , can you plug in both the cable modem and the local network?One approach is to install a second Ethernet card. The more economical approach: Connect the cable modem to the Uplink or WAN jack on your Ethernet hub or router. (On some models, an ordinary Ethernet port can be turned into an Uplink port if you flip a tiny switch.) Your gateway Mac plugs into the hub as usual.

19.7.2. The Software Base Station Effect

If the gateway Mac has an AirPort card, turning on Internet Sharing (and "To computers using: AirPort") has another profound effect: It creates a software base station. The Mac itself is now the transmitter for Internet signals to and from any other AirPort-equipped Macs within range. You just saved yourself the cost of an Apple base station!

Tip: Internet Sharing can create the opposite effect, too: Instead of letting a wireless Mac piggyback on a wired Mac's connection, you can let a wired Mac share a wireless connection.Suppose, for example, that you and a buddy both have laptops in a hotel lobby. You're online, using the hotel's WiFi network and your laptop's AirPort card. If you set up Internet Sharing appropriately, your buddy, who was too cheap to buy an AirPort for his laptop, can connect to yours via an Ethernet cable or even a FireWire cable ”and surf along with you!
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