To add an alias to a network device on freebsd do this:

[root@book etc]# ifconfig fxp0 alias 10.0.0.2 netmask 255.255.255.255

Note: The netmask is always /32

To remove one do this:

# ifconfig fxp0 -alias 10.0.0.2

To add these to rc.conf do something like:

ifconfig_fxp0="inet 10.0.0.1  netmask 255.255.255.240"
ifconfig_fxp0_alias0="10.0.0.2 netmask 255.255.255.255"