ifconfig alias

# cat /etc/hostname.dc0
inet 192.168.0.2 255.255.255.0 NONE media 100baseTX
inet alias 192.168.0.3 255.255.255.255
inet alias 192.168.0.4 255.255.255.255
Once you've made this file, it just takes a reboot for it to take effect. You can, however, bring up the aliases by hand using the ifconfig(8) utility. To bring up the first alias you would use the command:

# ifconfig dc0 inet alias 192.168.0.3 netmask 255.255.255.255

REFERENCE:
http://www.openbsd.org/faq/faq6.html#Setup.aliases