It is possible to create network alias or assign 2 ip address to single NIC under FreeBSD.

My setup:

lnc0 - IP : 192.168.1.1/255.255.255.0
lnc0 alias  - IP : 192.168.1.5/255.255.255.255

Note: Netmask must be different otherwise you will get an error

ifconfig: ioctl (SIOCAIFADDR): File  exists

A) From command line use ifconfig command as follows:

#  ifconfig lnc0 192.168.1.5 netmask 255.255.255.255 alias

B) You can setup this alias in /etc/rc.conf file by appending following text, so that next time FreeBSD comes up (after restart/shutdown) it will create alias for you:

ifconfig_lnc0_alias0="192.168.1.5 netmask  255.255.255.255"

C)Restart FreeBSD network service using following script:

# /etc/netstart

D) Display alias and real ip using ifconfig lnc0 command:

# ifconfig lnc0
lnc0: flags=8843 mtu 1500
inet6  fe80::20c:29ff:fe01:ddbd%lnc0 prefixlen 64 scopeid 0x1
inet 192.168.1.2  netmask 0xffffff00 broadcast 192.168.1.255
inet 192.168.1.5 netmask 0xffff

Source: http://ipucu.enderunix.org/view.php?id=1127&lang=en