This should be mind-numbingly simple and basic, and yet I couldn't find how to do it, either in "man ifconfig", in the OpenSolaris docs site, or Googling in desperation. I'm using OpenSolaris 2009.06.
When I try
ifconfig e1000g0:0 10.10.10.1 netmask 255.0.0.0 up all addresses go kablooie instead. This happen whether the network is in "automatic" or "manual" mode.
Alternatively, I could use a way to set up two static IP addresses too.