I know this...
Binding a new IP address to an interface. I know this. I do. Really. From the command line it's:
ifconfig fxp0 inet 192.168.144.120 netmask 255.255.255.255 alias
Then, in /etc/rc.conf I just add:
ifconfig_fxp0_alias0="inet 192.168.144.120 netmask 255.255.255.225"
Really it's so simple. But embarrassingly there has been the odd time I've mucked up. So I always like to check.