Each server must have a minimum of two interfaces set up, one interface for the actual device and another interface for a loopback used as an IP address alias.
server1% ifconfig ixgbe0 plumb server1% ifconfig ixgbe0 192.0.0.1 netmask 255.255.255.0 up
server1% ifconfig lo0:1 plumb
server1% ifconfig lo0:1 10.10.10.1 netmask 255.255.255.0 up
server1% ifconfig -a
lo0:1:flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
inet 10.10.10.1 netmask ffffff00
ixgbe0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 3
inet 192.0.0.1 netmask ffffff00 broadcast 192.0.0.255
ether 0:1b:21:53:6e:e0
server1% route add 20.0.0.0/24 192.0.0.100