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Advanced Networking for Mactintosh Professionals


User Guide


IPNetRouter Guide to AirPort

  1. Introduction to AirPort and IPNetRouter
  2. Hardware versus Software Base Station
  3. Base Station versus Computer-to-Computer Mode
  4. Improving IPNetRouter and AirPort Compatibility
  5. Using IPNetRouter to Extend Your AirPort Configuration
  6. Additional AirPort Information

Introduction to AirPort and IPNetRouter

The AirPort Software Base Station (Access Point or AP) uses a licensed version of IPNetRouter (IPNR) to configure IP multilink multihoming and forwarding in Open Transport, and also to provide the NAT and DHCP Server functions. This page provides an overview of how the AirPort Software Base Station works and how you can use IPNetRouter in wireless networking configurations.

Identifying the Pieces

The AirPort software includes a Faceless Background (FBA) version of IPNetRouter. "AirPort AP" in the Extensions folder is the application, and "AirPort AP Support" is the proxy module corresponding to OTModl$Proxy [use Conflict Catcher to get information about these files]. Notice that installing the AirPort software will remove OTModl$Proxy and replace it with a possibly older version named "AirPort AP Support". To keep things as synched up as possible, we recommend using the latest IPNR and Airport updates available.

Why You Might Want to Use IPNetRouter With AirPort

While the Software Base Station is included free with AirPort, its configuration options are limited to keep things simple. IPNetRouter gives you the flexibility to connect separate LANs using AirPort wireless and support additional networking devices and IP subnets. We (Sustworks) support IPNetRouter and can help you with configuration questions or bug reports.

Some people use IPNetRouter to connect their LAN to the Internet and then use one or more Hardware Base Stations (HBS) in transparent bridging mode to extend their LAN to any wireless clients. This configuration may offer the best combination of performance and flexibility and was used to provide Internet access to hundreds of developers at MacHack 2000 for example.

Another configuration used an AirPort capable Mac to connect with a remote Office LAN (with a range extending antenna), used IPNetRouter to share this connection with their Home LAN, and then used an HBS to extend the home LAN to wireless clients that were out of range of their office.

When configuring IPNetRouter to use AirPort wireless, you can treat the wireless card just like another Ethernet.

Getting Help with AirPort

AirPort is an Apple product supported by Apple. If you are looking for help with AirPort that does not involve the configuration of our IPNetRouter software, please consult the AirPort documentation, check public forums like MacFixit, or contact Apple. If you would like help deciding whether IPNetRouter might be the best wireless routing solution for your network, we welcome your questions.


Hardware versus Software Base Station (Access Point)

The AirPort Hardware Base Station is running a version of KarlBridge, which was developed by Doug Karl at Ohio State <>. Our software is not part of this implementation.

The AirPort Software Base Station uses a licensed version of IPNetRouter developed by Peter Sichel <>.

What are some of the differences? That is, why would you choose one over the other?

  1. The hardware AP is $299 whereas the software AP is included with Apple's $99 AirPort PC card but requires an AirPort capable Mac. The full UI version of IPNR is $89 if purchased separately.

  2. With the hardware AP, everyone else doesn't lose their connection if you have to reboot your Mac.

  3. The software AP does not support either AppleTalk or transparent bridging mode. It does IP routing with NAT and DHCP.

  4. The software AP (IPNR) allows you to connect two Ethernet LANs via wireless. Hardware base stations cannot connect to each other in this way because they only operate in Base Station mode (there is no way to set Computer-to-Computer mode, or use one Base Station as a wireless client of another).

  5. The software base station supports Trace Route (NAT with ICMP translation), can work with PPTP, and has various other implementation differences. It has the potential to offer higher performance and more flexibility (multiple Ethernets, PPPoE, LocalTalk, or any other Data Link Provider).

Apple will no doubt continue to enhance both products.


Base Station versus Computer-to-Computer mode

Apple's Hardware AP operates in Base Station mode only, whereas the AirPort software in combination with IPNetRouter can set the radio to either Base Station, Client, or Computer-to-Computer mode (802.11 "ad-hoc" mode).

Base Station mode is similar to a cellular network and provides relaying and power saving features. The Base Station can buffer traffic for low power clients allowing them to periodically turn on their radio and ask the base station for any traffic that may be waiting. By switching on the radio only long enough to receive any waiting traffic, clients can save precious battery power by not operating the radio all the time.

In Computer-to-Computer (ad-hoc) mode, the radio listens for incoming traffic all the time. Computer-to-Computer mode is intended for setting up ad-hoc limited range LANs when no base station (or cellular network of base stations) is available. As a side effect, it may offer better network performance, but consumes more power. We suggest using this mode with IPNetRouter if possible for best performance and stability.

By setting the AirPort Software to use Computer-to-Computer mode along with IPNetRouter, it is possible to connect multiple Ethernet LANs via AirPort wireless. AirPort wireless is just another data link provider logically equivalent to Ethernet.

Since Apple has not published the commands to switch the radio between Base Station and Computer-to-Computer mode, the only way to select Base Station mode is to use the AirPort Setup Utility (and then optionally modify the configuration or switch over to running the UI version of IPNetRouter) as described below.


Improving IPNetRouter and AirPort Compatibility

Apple's Software Base Station (SBS) uses a licensed version of IPNetRouter supplied as a Faceless Background Application (FBA). Understanding this can help you avoid potential conflicts between AirPort and IPNetRouter. The following instructions apply to IPNetRouter 1.4.7 or later and AirPort 1.2 or later. We recommend that you install/update to AirPort 1.2 or later to minimize conflicts between AirPort and IPNetRouter.

Installing the AirPort software may remove IPNetRouter's OTModl$Proxy shared library and replace it with a possibly older version named "AirPort AP Support". Also note that Apple usually updates Open Transport itself with AirPort updates. You may need to reinstall IPNetRouter, IPNetMonitor, IPNetSentry and/or other third party networking software (like PPPoE client software) after installing a software update of any Apple AirPort software.

[Tim] In order to futher minimize conflicts between AirPort and IPNR I recommend that you use IPNetRouter in Computer-to-Computer mode unless you have a good grasp of how AirPort works. It avoids the complications that can occur if your AirPort cards fall asleep or you accidently attempt to enable software base station mode with our software installed. I use this method on my own home office network quite effectively. Before I did this I had all sorts of problems!

Before installing IPNetRouter or while IPNetRouter is not running on the gateway Mac:

  1. Set Computer to Computer mode on the gateway Mac for AirPort.
  2. Verify that Appletalk works between the gateway and another wireless Mac using Filesharing. If it doesn't, don't proceed further until you can get this to work without IPNetRouter before proceeding.
  3. Open the AirPort Apple menu item
  4. Using the Software base station button, verify that your gateway machine is not in software base station mode. If it is running, turn this off and then close the "AirPort" window.
  5. Next, open the Extensions Manager control panel.
  6. Disable the "AirPort AP" and "AirPort AP Support" files. (These are the equivalent of the IPNetRouter FBA and "OTModl$Proxy" extensions. Disabling these extensions does not prevent AirPort from working but will prevent potential conflicts with certain files created when Software Base Station mode is configured on the IPNetRouter gateway machine; the files involved are explained elsewhere in the AirPort section of the Guide.)
  7. If the "AirPort AP Configuration" file is present in your system's Preferences folder, remove it (do not delete it if you might want to switch back to a software base station configuration you've used successfully in the past--always a good rule!).
  8. Reboot the gateway machine.
  9. Begin configuring IPNetRouter as you normally would for an Ethernet interface. See elsewhere in the IPNetRouter Guide for typical ethernet configurations.


Using IPNetRouter to Extend Your AirPort Configuration

AirPort AP (Apple FBA version of IPNR) is configured using the AirPort Setup Utility. Turning on the Software Base Station writes out an IPNR configuration file named "AirPort AP Configuration" in your Preferences Folder, and then sends an Apple Event to launch AirPort AP. Any changes you make to the Software Base Station writes out a new "AirPort AP Configuration" file. To use IPNetRouter to extend your AirPort configuration, you have three choices:

  1. Quit the AirPort AP FBA and launch the UI version of IPNetRouter to modify your IP network configuration directly.

  2. Modify AirPort AP to use a different configuration file that you specify (STR# resource 131 Option Settings, "ConfigFileName=Airport AP Configuration"). [Note this technique is for registered IPNR users who wish to integrate their use of IPNR with AirPort. Proceed at your own risk since the AirPort Setup Utility could produce unexpected results.]

  3. Use our FBA instead of Apple's. In this case, disable the AirPort AP files in your system's Extensions and Preferences folder.

To verify whether your AirPort AP configuration succeeded, you can examine the "AirPort AP.log" file in your Preferences Folder.


Additional Airport and Wireless Information

Apple has updated Airport Software and hardware firmware (for the hardware basestation) several times since they were first released in 1999. Apple has also issued numerous technotes on some of the limitations and other idiosycracies of Airport hardware and software performance and configuration since its introduction. To see all the technotes with regard to Airport, use Apple's search engine. See also the Troubleshooting section.

If you are looking for information about wireless LAN solutions other than Airport, a good place to start is our web based nettalk archives search engine.

Disclaimer: The information above is from existing published sources or otherwise readily available.