Single Link Multi-homing System Setup

Excerpt from "Open Transport 1.3 Technical Information"
©1997 Apple Computer. Inc.

Open Transport 1.3 introduces Single Link Multi-homing, a mechanism by which Open Transport can support multiple IP addresses on the same hardware interface. Synonyms for this feature include IP Aliasing, Secondary IP address Support, IP Masquerading, "Multihoming", IP Multinode support. This is useful for sites like Internet Service Providers (ISPs), that want to give each of their clients a distinct IP address, without requiring separate computers for each address. Web server software packages, or server plug-ins that utilize this feature can offer virtual domain support that supports all web browsers. You will need to contact your application developer to see if they are supporting this new feature of Open Transport. This feature is only available with Open Transport 1.3 and later.

You configure a system to use multiple IP addresses as follows:

1. The TCP/IP Control Panel must be set for manual addressing.

2. You create a text file with the required name "IP Secondary Addresses" and put it into the Preferences folder in the System Folder.

Each line of the IP Secondary Addresses file contains a secondary IP address to be used by the system, and an optional subnet mask and router address for the secondary IP address. If there is no subnet mask entry, then a default subnet mask for the IP address class will be used. If there is no router address entry, then the default router associated with the primary address will be used.

Each secondary address entry must be prefixed by "ip=". Each subnet mask entry must be prefixed by "sm=". Each router address entry must be prefixed by "rt=". Lines proceeded by a ; are ignored. An example of the contents of the IP Secondary Addresses file follows.

; 'ip=' for ip address, 'sm=' subnet mask, 'rt=' router address
;  Note: no space in 'ip='
; IP address             Subnet Mask            router addresses
;-----------             -----------            ----------------
ip=        sm=       rt=
ip=                               rt=

The order of the entries is important. The "rt=" entry must follow the "sm=" entry if used.

When Open Transport activates TCP/IP, the primary address will be obtained from the TCP/IP Control Panel setting. Open Transport then looks for the IP Secondary Addresses file in the Preferences folder to determine if additional addresses should also be configured. If there are duplicate IP address entries in the IP Secondary Addresses file, the duplicate addresses will be ignored. When Open Transport binds a TCP/IP connection, if there is an address conflict with either the primary or any secondary addresses with another host, Open Transport will present an error message using a dialog box and unload Open Transport/TCP from memory. The error dialog will display the conflicting IP address, the hardware address of the conflicting machine and note that your TCP/IP network interface has been shut down.

To resolve the conflict, quit all of the TCP/IP applications on both conflicting machines, reconfigure TCP/IP on one of the machines so there is no longer an address conflict, then relaunch your TCP/IP applications.

©1997 Apple Computer. Inc. Apple, the Apple logo, AppleShare, AppleTalk, LaserWriter, LocalTalk, MacTCP, Performa, Power Macintosh, PowerBook, and PowerBook Duo are trademarks of Apple Computer, Inc., registered in the U.S. and other countries. NuBus is a trademark of Texas Instruments. PowerPC is a trademark of International Business Machines Corporation, used under license therefrom. All other product names are trademarks or registered trademarks of their respective holders. Mention of non-Apple products is for information purposes and constitutes neither an endorsement nor a recommendation. Apple assumes no responsibility with regard to the selection, performance, or use of these products.

Page last updated 12/28/1999