UInt16 index = 0;
Err err = errNone;
while (err == errNone)
{
    UInt16 idx;
    UInt32 creatorID;
    UInt16 instance;
	
    //Retrieve the creator ID and instance of the indexed Interface
    err = NetLibIFGet(netLibRefNum, index, &creatorID, &instance);
    if (err == errNone)
    {
        //Check to see if the interface is up
        UInt8 isUp = false;
        UInt16 size = sizeof(isUp);
        NetLibIFSettingGet(netLibRefNum, 
                           creatorID, 
                           instance, 
                           netIFSettingUp,
                           &isUp,
                           &size);
			
        //Make sure the interface is up and is not the 
        //loopback interface
        if (isUp && creatorID != 'loop')
        {
            //Once we get here, it's simply a matter of 
            //grabbing the IP address
            UInt32 address;
            size = sizeof(address);
            NetLibIFSettingGet(netLibRefNum, 
                               creatorID, 
                               instance,
                               netIFSettingActualIPAddr, 
                               &address, 
                               &size);

            //Once you get here, you have to do something 
            //with the IP address.  Do you just get the 
            //first one and stop, or do you want all the
            //assigned IP addresses of Up Interfaces?  
            //That determines whether or not you want to 
            //break out of this loop.  I leave that as an
            //exercise for the reader.  -Jon
        }
     
        //Go on to the next Interface
        index++;
    }
}