Using a Mac Running OS X to Share an Internet Account

Mac OS X includes a built-in DHCP server you can use to share a single Internet connection with other devices on your local network. And if the Macintosh on which you configure the DHCP server includes an AirPort card, you can also provide a wireless AirPort network without the use of an AirPort Base Station.

NOTE

The function of the DHCP server is to provide and manage IP addresses to devices on the network. The DHCP server doesn't actually provide the Internet access itself; that comes from the connection method you use (such as a cable modem). The DHCP server manages the traffic between the Internet connection and the devices on the network.

One advantage of this approach is that you don't need to add dedicated Internet sharing hardware (such as a sharing hub or an AirPort Base Station) to your network; a standard Ethernet hub will enable you to share an Internet account over an Ethernet network, and an AirPort card will enable you to share an account over a wireless AirPort network. Another advantage is that it doesn't cost anything to share an account (assuming that you already have the connection hardware, such as for an Ethernet network).

NOTE

You can use Mac OS X's built-in firewall to protect the DHCP machine from attacks from the Internet (and because it sits between the Internet and the other devices on your network, it protects those devices as well).

To learn how to configure the Mac OS X firewall, see "Defending Your Mac Against Net Hackers," p. 803.

This approach does have one significant disadvantage and one minor drawback, however. The significant disadvantage is that the Mac that is providing DHCP services must always be running for the machines that use it to be able to access the Internet. If the DHCP machine develops a problem, no device on the network will be able to access the Internet. The less significant issue is that the DHCP services do require some processing power. Providing these services will most likely not result in any noticeable performance decrease, but if your machine runs at its limits, asking it to provide these services might slow down other tasks slightly.

NOTE

DHCP servers are not platform specific. For example, if you have a DHCP server running on a Macintosh, you can connect a Windows computer to the network and use the same DHCP server to share the Internet account with it. Or, you can install a DHCP server on a Windows machine and use it to share the account with Macs on the network.

Configuring a Mac to provide DHCP services to a network requires the following general steps:

1. Connect the Mac to the Internet.

2. Install the network you will use to connect the Mac with other machines.

3. Configure the Mac to share its Internet account.

Connecting the DHCP Mac to the Internet

It goes without saying (but I will say it anyway) that in order to use a Mac to share an Internet account, that Mac must be connected to the Internet. The method you use to connect to the Internet doesn't matter. You'll get the best results if you use a broadband connection, such as a cable DSL modem, but you can also share a dial-up connection if you want (don't expect speedy operation, though).

To learn how to connect a Mac to the Internet, see Chapter 10, "Connecting Your Mac to the Internet," p. 233.

Installing the DHCP Mac on a Network

The next step is to install the DHCP Mac on the network you are going to share the Internet connection with. You can build a wired network using Ethernet, or you can install an AirPort card to connect the Mac to other AirPort-equipped Macs (you can share an account with other machines using both networking methods at the same time).

To learn how to install, configure, and use an AirPort network, see Chapter 11, "Using an AirPort Network to Connect to the Internet," p. 265.

To learn about Ethernet, see "Ethernet," p. 625.

To learn about Ethernet hubs, see "Finding and Installing an Ethernet Hub," p. 710.

To learn how to build and manage a network, see Chapter 25, "Building and Using a Network," p. 721.

Configuring the DHCP Mac to Share an Account

After you have configured the Mac for Internet access and connected it to other computers (with or without wires), you need to configure the Internet sharing services on it.

There are three possibilities when you configure Internet sharing on your Mac:

• Your Mac is connected to the Internet via an Ethernet connection, and it also has an AirPort card installed in it.

• Your Mac is connected to the Internet via Ethernet and it does not have an AirPort card installed in it.

• Your Mac is connected to the Internet via an AirPort Base Station, in which case you can share the Internet account by connecting that Mac to other computers to share its account via an Ethernet network.

When you configure Internet sharing on your Mac, it automatically determines which case is true for your machine and will present the appropriate options for you. To configure Internet sharing, use the following steps:

1. Open the System Preferences Utility and click the Sharing icon to open the Sharing pane.

2. Click the Internet tab. What you see depends on how the Mac is connected to the Internet. For example, in Figure 26.1, you see the Sharing Internet tab for a machine that is connected to the Internet via Ethernet and that does not have an AirPort card installed in it. In Figure 26.2, you see an example of a machine that is connected to the Internet via an AirPort Base Station.

   Figure 26.1. This Internet Sharing tab is for a computer connected to the Internet via Ethernet.

   

   Figure 26.2. This Internet Sharing tab shows a machine that is currently connected to the Internet via AirPort.

   

3. If you see a "Share the connection…" check box, check it; if the Start button is already active, skip to Step 5. You might see a dialog box warning you that activating sharing might cause problems for other ISP customers or might violate your service agreement (some providers prohibit sharing an individual account on multiple machines).

4. Read the dialog box and click OK to close it. The Start button should become active.

5. Click Start. Your connection will be shared with all the devices your Mac can communicate with. For example, if your Mac is connected to a local network via Ethernet, other devices on the network can use the account via the DHCP services your Mac provides.

6. Quit the System Preferences Utility.

7. Configure the other devices on the network to use the Mac's DHCP server to access the Internet.

To learn how to install, configure, and use an AirPort network, see Chapter 11, "Using an AirPort Network to Connect to the Internet," p. 265.

To learn how to connect a Mac to the Internet, see Chapter 10, "Connecting Your Mac to the Internet," p. 233.

TIP

If you share an Internet account over AirPort, an upward pointing arrow is added to the center of the AirPort icon in the menu bar. This indicates that the connection is shared and also indicates that you can access the sharing controls from the menu.