Here is how you setup a IP alias in FreeBSD. Add the following to your rc.conf file.
ifconfig_rl0_alias0="192.168.0.57 netmask 255.255.255.255"
Now here is the command you can use to bring that interface online instantly.
ifconfig rl0 alias 192.168.0.57 netmask 255.255.255.255
With BSD you use 255.255.255.255 for you IP alias netmasks.