Operating with aliases on Ethernet interfaces

Adding and removing an alias manually
ifconfig fxp0 inet 10.0.0.232 netmask 255.255.0.0 alias
ifconfig fxp0 inet 10.0.0.232 -alias

Set up an permanent alias
ifconfig_fxp0_alias0="10.0.0.232 netmask 255.255.0.0"