Very obvious problem
I have Realtek card and want to do IP alias

Consider Following Example
I have Realtek card having
Primary IP : 192.168.1.1
Want to add IP alises 192.168.1.15 and 192.168.1.25

Where to add - Update /etc/rc.conf

ifconfig_rl0="inet 192.168.1.1 netmask 255.255.255.0"
ifconfig_rl0_alias0="inet 192.168.1.15 netmask 255.255.255.0"
ifconfig_rl0_alias1="inet 192.168.1.25 netmask 255.255.255.0"

Bringing up the new IPs:

# /etc/rc.network restart

NOTE THAT Please take extra caution while restartiing the network