Multihoming a Network Interface on Mac OS X Server

—Joel Rennich,

1 June 2002—Updated 10 November 2002

Network Interface Multihoming This process remains effectively the same under Jaguar as under 10.1.

It’s pretty easy to set up Mac OS X Server with multiple IP addresses when it has multiple network interface cards, but what about when you need multiple IP addresses on the same ethernet card? Until recent builds of Mac OS X this used to be a lot harder; now it’s really easy.

Launch System Preferences and go to the Network Preference Pane. In the Show pull-down menu select “Active Network Ports.” Select one of your interfaces and click on the Duplicate button just to the left of it. Now you have two of that interface. You can configure both of these as you normally would any other interface, but keep two things in mind:

1. If you are multihoming on the same subnet, make sure that the second IP address has a Subnet Mask of 255.255.255.255. This will ensure that you only route out on one IP address and save you lots of problems down the road. In my experience 10.2 is smart enough to figure this out for you so you should be able to disregard this.
2. Secondly you need to make sure that only one of your duplicated interfaces has AppleTalk active. This also applies if you have multiple interfaces connected to the same physical network. If AppleTalk is active on more than one interface your computer will see its own AppleTalk name already on the network from the other interface. This will cause Mac OS X to shut down AppleTalk on all interfaces, since there can’t be two machines with the same name on the network. The system will actively alert you to the fact that AppleTalk can’t run on both interfaces at the same time.