IP addresses, multiple
ifconfig_rl0_alias0="192.168.0.57 netmask 0xffffffff"
At the command line:
ifconfig rl0 alias 192.168.0.57 netmask 0xffffffff
If you want the virtual interaface to come up when the computer boots, you'll need to place the line in rc.conf, although if you're just changing things on the fly (for testing etc) and won't mind that the new address goes bye-bye on a reboot, just doing the command line ifconfig with alias is enough. This is more easily done through webmin, and can come in real handy for serving different webpages from the same machine.
Note that running multiple IPs on different nets will put your interface into promiscuous mode. (ifconfig or ip link may report this).