Mac OS - Share an Internet connection

STEP 1. Open the TCP/IP control panel. Your standard configuration (possibly entitled "Default") for connecting to the Internet will be displayed. Press - to open the "Configurations" dialogue box. Select your standard configuration and click on Rename. Change it to "IPNetRouter" (making sure you get the case right). Then, with "IPNetRouter" still selected, click on Duplicate. Then select "IPNetRouter Copy" and click on Rename. Call the copy "Ethernet LAN" or something equally descriptive of your home network. Then click on Make Active to set this as the default configuration, and close the Configurations window.

STEP 2. The TCP/IP control panel should now be displaying the configuration called "Ethernet LAN". In the "Connect Via" pop-up menu, select Ethernet.

STEP 3. It's time to give your Inter-net gateway an IP address. Don't just make one up, and, whatever you do, don't use one you've read elsewhere (your ISP, for instance). Addresses must be unique. However, addresses in the range 192.168.x.x have been reserved (by the people who reserve such things) for private intranets. So, if you give your network an address in this range, you won't be clashing with anyone.

In the TCP/IP control panel, enter the IP address "192.169.0.1" as the IP address for the Ethernet LAN configuration. Enter the subnet mask "255.255.255.0". Leave the router address empty, leave the name server address the way it was configured to work with your ISP, and close the control panel, saving your changes.

STEP 4. Launch the IPNetRouter application. The "Interfaces" window should open, showing the Ethernet configuration you just created in the "Port Name" column. Under "Configure Interface", select PPP from the pop-up menu, then click on Add. Make sure your modem is plugged in, as your computer will now attempt to establish a PPP connection and obtain an IP address from your ISP. If you get an error message or if PPP fails to make a connection, go back to the TCP/IP control panel and make sure that your PPP configuration is correctly titled: it must be called "IPNetRouter".

STEP 5. The "Routes" window will now open, showing a number of "Direct" routes and one "Default-Gateway". If you have more than one "Default-Gateway", there is a problem. Possibly you had some numbers entered in the router address field of the TCP/IP control panel. Go back and clear this field if that is the case.

STEP 6. Your Mac is now playing host to multiple IP addresses and acting as a gateway to the Internet - a trick called "multi-homing". This is very cool, but fairly useless. To allow computers on your LAN to share your PPP connection, you have to fool the Internet into believing that all the traffic coming from that gateway address is coming from a single machine. In the "Interfaces" window, select PPP in the "Port Name" column, then check "IP Masquerading" and click Add. A little mask appears next to "PPP".

STEP 7. This is the tricky bit. You have to configure each of your other machines' TCP/IP stacks to connect to the Internet using this gateway. You can, however, get any computer with TCP/IP to share your connection, whether it's Windows, Linux or whatever. The IP address should be "192.168.0.x", where x is a unique number (especially not 1). The subnet mask is "255.255.255.0" and the router address is the IP address of your gateway Mac, "192.168.0.1". You needn't have anything in the "name server address" field, but you may wish to enter the address supplied by your ISP.

STEP 8. Back to your gateway Mac, and the IPNetRouter application. From the File Menu, select "Save As . . ." and save your IPNetRouter configuration. Give it a name like "Shared Internet". Then quit IPNetRouter. Find the configuration file in the Finder, and make an alias of it (-). Drag this alias into the "Apple Menu Items" folder in your System folder. Quit IPNetRouter.

You're ready to go. If you want to start an Internet session that isn't shared, simply connect as normal using Remote Access. Or, if you want to start a shared connection, select the "Shared Internet" alias in the Apple menu and IPNetRouter will launch automatically.

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