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Sharing Your Internet Connection with an Older Mac

This document describes how to connect an older Mac to the internet by sharing the connection that you use with your modern Mac or PC. Some Mac network configurations are easy to set up and you'll be connected to the internet in minutes. Others may take more effort but hopefully you'll find the answer to your questions in this document or the associated links. treat this document as a technical supplement to the instructions from your ISP or network hardware supplier rather than a recipe.

You may find it helpful to refer to two other guides.

Old Macintosh System Software and TCP/IP describes what software you need to install to use TCP/IP on a Mac. The guide covers System 6 through to Mac OS 9.x. It includes specific information on configuring MacTCP.

Old Macintosh Networking Primer describes the components that make up an ethernet network and some TCP/IP basics.

Let's start off with a few definitions.

An Internet Connection is any sort of network link from your home or office to the Internet. The connection can be a dial-up modem, cable modem, ADSL/DSL, ISDN or a fixed link. It is assumed that there is a working ethernet connection between the network device or computer that actually connects to the Internet and your older Macs.

An older Mac refers to any Macintosh that your ISP (Internet Service Provider) is unable or unwilling to support. Many ISPs seem happy with Macs running OS 8.5 or higher but their staff are often unfamiliar with earlier systems. It would be very unusual for there to be a genuine technical reason why you should not be able to connect a Mac running System 6 or higher through your shared connection. Ignore the negative "won't work" comments from the technical support desk at your service provider -- such comments are more often dictated by "support policies" than by potential or real technical problems.

You and Your ISP

Check the terms and conditions of your connection. Your ISP may not expect you to run an ftp or web server on a domestic broadband connection or it may be explicitly forbidden. Some TCP/IP ports may be deliberately blocked to "prevent" use of prohibited services.

Make sure that you are familiar with the documentation (paper and on-line) supplied by your ISP about your connection and any networking devices they provide. Even if you don't understand everything on their pages, know where to find them. If you're confident that you understand what you've got, search the web and newsgroups for information provided by other users of your ISP and network devices.

Don't worry if your ISP connection is designed for a single computer. Your older Macs normally won't make much impact on your bandwidth consumption. However it may not be politically wise to tell your ISP that you are sharing the connection to your older Macs if you ever have to contact them with a technical problem...

Your Current ISP Connection

Broadband Connections

If you have a broadband connection, your ISP will have provided or advised you to purchase some sort of connection device or "black box". Some common types of broadband black box are:

There are many providers of black box and I have no intention of trying to include detailed information about particular devices. It is worth noting, however, that some unbranded black boxes are simply rebadged devices from manufacturers such as LinkSys. If you need more technical information about an unbranded black box, look at the FCC ID code label and trace the original manufacturer at

Dial-up Modem Connections

If you intend sharing a dial-up modem conection, make sure that you have a good quality V90/56kbps or V92 modem that reliably connects to your ISP. As a general recommendation, you should purchase a traditional, general purpose modem rather than a winmodem. With a general purpose modem, of the essential modem functionality is built-in, whereas a winmodem (or software modem) uses fewer chips and emulates the missing chips in software. Winmodems generally require that you are running Windows (a few have drivers for Linux) and connections are slower when used with an older PC because the computer has to do more work. Most cheap internal modems based on a PCI or ISA expansion card are winmodems and most external modems are general purpose models. This simple rule is not always true, however, so check the specification carefully before buying.

Apple's own internal modems for the G3 and iMac onwards generally work well. Reliable general purpose external modem brands include US Robotics/3Com, Pace, Zoom etc but not all models work equally well. Apple's external Geoport modems are old and slow, but are easy to configure.

When using an external modem you will require a serial cable to connect it to your modern computer. Many modems are supplied with a PC serial cable so you may need to buy one for your Mac. Again, check the cable specifications and ask whether the Mac cable supports hardware hand shaking.

Some manufacturers of networking kit have built modem/router boxes that are specifically designed for sharing a dial-up connection to several computers. These devices worked well according to the reviews but never sold in great numbers. A more popular option is to run internet connection sharing software on a reasonably powerful computer.

Two sites provide solid advice on the latest modem standards: and

If you need to make your own Mac serial cables, visit

Network Address Translation (NAT)

The internet was designed on the basis that every computer connecting to it would have a unique "public" IP address. A public IP address is one that can be directly accessed from any other computer on the internet. For organisations that wish to use the TCP/IP protocol on internal networks, the standards authorities also created three "private" IP address ranges. A private IP address can only be directly accessed by computers on the same network and you will use private IP addresses to connect your older Macs to your ISP. Network Address Translation is a popular mechanism for connecting computers with private IP addresses to other computers on the public internet.

The rapid growth of the internet coupled with the slow transition from IPv4 to IPv6 means that public IP addresses are scarce. If your ISP provides you with a public address, they will charge if you want more than one. Some ISPs may only provide you with a private address which is connected to the internet via your ISP's NAT service. Fortunately, having a NAT /private address from your ISP is not a problem because you can use a second NAT service in your home/office to connect several computers to your ISP.

If you have a public IP address and your ISP allows you to run a web or ftp server, a good NAT service will allow you to redirect other computers on the internet to a computer with a private IP address on your local network. Basic internet connection sharing software may not be capable of this but any intelligent modem router box will provide this functionality. If you only have a private IP address from your ISP, you'll have to do your own homework to determine whether other users of your ISP have solved the problem.

Before trying to share a single network connection, determine whether your ISP has assigned you an IP address from the public address ranges or the private address ranges. The private IP address ranges are:

If your ISP has issued a private IP address to you, it means that they have implemented their own NAT service to reduce the number of public addresses that they use. You should not use an address range for your own private network if it is already used for NAT by your ISP. For example, if your ISP has assigned you, do not use addresses in the range --> on your own network because they may already be used by other customers of your ISP. If you do configure NAT using the same address range as your ISP, expect to have your connection cut off if your internet connection sharing software starts assigning invalid IP configurations to other customers. Always use addresses from a different private address range than your ISP.

For more technical information on NAT, refer to

Internet Connection Sharing Software

You will need to use internet connection sharing software if you are connected to your ISP by:

Note that you do NOT need to run internet connection sharing software if your internet access is provided by an intelligent modem router with more than one ethernet interface.

Internet connection sharing software which incorporates NAT runs on the computer that is connected to the internet. The software can optionally be configured to dial-up your ISP on demand if you are using a PPP or serial modem connection. Most internet connection sharing software includes a DHCP server to assign IP addresses to computers that share the connection.

If you have a simple broadband modem (one with a single ethernet port), your Mac or PC will need two ethernet cards if you intend using internet connection sharing software. The first card should be configured using your ISP's instructions for connecting to the intenet; the second card should be configured using the instructions for your internet connection sharing software.

Internet connection sharing software exists for most modern operating systems. The most popular are briefly described below.

Mac OS X

From OS X 10.2 onwards, Apple have provided a simple interface to enable internet connection sharing. It provides a DHCP server and allocates IP addresses in the range --> ( is the Gateway address). The graphical user interface does not provide a way to change this IP address range so if your ISP has assigned you an address from -->, do not use the Mac OS X defaults.

To change the NAT/DHCP address range, you will need to edit the configuration files manually (try a Google search for "configure natd ipfw Mac OS X") or use a graphical tool such as IPNetShareX from Sustainable Softworks.

Mac OS 7 on with Open Transport

The most popular NAT/DHCP option for Macs running System 7.5.3 or higher is IPNetRouter from Sustainable Softworks. Versions are available that run on any Mac with a 68030, 68040 or PPC processor and the online documentation is well written. Building a Cross-Platform Home Network ( contains further information about IPNetRouter.

Two alternatives are Vicom SurfDoubler or Vicom SoftRouter/Internet Gateway but both require a PowerPC and System 7.6.1 or later.

Other Mac OS

Apple IP Gateway (any 68020 or greater Mac, System 7.1 or greater) is an old piece of software that originates from the days before Open Transport. It is no longer available commercially but would be interesting to play with on a retro network.


Recent versions of Windows (98SE, ME, 2000, XP, Vista) include basic internet connection sharing software. The name is often shortened to ICS. For once, the Windows help system does a good job of describing how to configure the software. As always, ICS differs for different versions of Windows but all implementations include NAT and DHCP.

The Windows DHCP servers allocate addresses in the range --> so if your ISP has assigned you an address from -->, do not use the Windows defaults. For Windows 98SE and Me, refer to Microsoft Knowledge Base Article 230148 "How to Change the IP Range for the ICS DHCP Service". I have not been able to find a similar resource for Windows XP or 2000.

Third party connection sharing products exist for Windows that you might use as an alternative. I've never used any of them. Three Macs and a Printer ( maintains a list of Windows products.

If you are running Windows 95, 98 or Me and you need to know your ISPs gateway and DNS addresses, type WINIPCFG in the Run dialog on the Start menu. A graphical application will start up; select the network interface that the PC uses to connect to your ISP.

If you are running Windows NT, 2000, XP or Vista and you need to know your ISPs gateway and DNS addresses, type WINIPCFG in the Run dialog on the Start menu. Type IPCONFIG /ALL |MORE at the command prompt. A text display of the configuration of all network interfaces will be shown. Note that the | symbol is called a pipe and is found on the \ key on US and UK keyboards.

Linux or BSD

Connection sharing is built into the operating system. Some distributions may include a graphical tool for configuring NAT but you'll probably need to edit the config files manually.

Sharing an Intelligent Modem Router

In theory, sharing an intelligent modem router with several utp ethernet ports and built-in NAT/DHCP servers is a very straightforward procedure. You do not need to configure internet connection sharing software on your modern computer with this hardware.

Problems that people have reported include:

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