Using Multiple IP Addresses to Share an Internet Account

With some broadband accounts, such as cable, you can purchase additional IP addresses (or DHCP names) so that you can configure multiple machines to access the same account. The requirements to do this are an address for each device that will be using the account, and an Ethernet or other network, which typically consists of a hub with the cable or DSL modem connected to the WAN port and each device connected to a LAN port.


Some ISPs use DHCP to provide IP addresses to you. In these cases, you assign a DHCP name to your computer. The ISP's DHCP server assigns IP addresses to you as you need them. To share an Internet account among several devices, you need a unique DHCP name for each device; you use this DHCP name to configure the network on each device.

To share an account using multiple IP addresses, do the following:

  1. Contact your ISP to determine whether this option is available.

  2. If it is, obtain additional addresses (or DHCP names); you will need one address for each machine or device (such as an AirPort Base Station) that you want to share the account. You will probably need to pay an additional fee for each IP address you obtain.

  3. Connect the cable or DSL modem to the WAN port on the hub for your network.

  4. Connect each device to a LAN port on the hub.

  5. Configure each machine with one of the available addresses (or DHCP names).


If you can't use a device on your network because of an error message about the same IP address being used on more than one device, see "I Get an Error Message Telling Me That Multiple Devices Have the Same IP Address" in the Troubleshooting section at the end of this chapter.

One advantage to this method is that you can use a standard Ethernet hub to facilitate sharing the account; these hubs are quite inexpensive and are simple to install and use. And setting up each device to use its address is straightforward as well. You simply configure each machine as if it were the only one using the account. Another advantage is that you get maximum speed for each machine because each connects directly to the account; the traffic doesn't have to be managed by a DHCP server on your network. And, because there isn't any software on your network that has to manage the Internet traffic, as a DHCP server does, your connection for each device is only dependent on your modem and service, making it more reliable than some of the other methods.

One possible disadvantage is that might you have to pay an additional fee for each address you use. The typical cost of additional addresses is $5 to $7 per month per address on top of the address included with your base account. This can get expensive if you have several devices on your network; however, you can balance that cost against not needing a hub (Ethernet or AirPort) that has Internet account sharing built in.

Another disadvantage is that you don't get any special features, such as a built-in firewall. You will have to add protection for your network in some other way.

For help choosing and installing an Ethernet hub, see "Finding and Installing an Ethernet Hub," p. 710.

For help installing and configuring a network, see Chapter 25, "Building and Using a Network," p. 721.

For help with protecting your Mac from Net attacks, see "Defending Your Mac from Net Attacks," p. 803.

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