TCP/IP Alias Add/Delete

Temporarily add a default route
route add default 192.168.1.1

Temporarily add an address to an interface
ifconfig en0 192.168.1.2 netmask 255.255.255.0

Temporarily add an alias to an interface
ifconfig en0 192.168.1.3 netmask 255.255.255.0 alias

To permanently add an IP address to the en1 interface
chdev -l en1 -a netaddr=192.168.1.1 -a netmask=0xffffff00

Permanently add an alias to an interface
chdev -l en0 -a alias4=192.168.1.3,255.255.255.0

Remove a permanently added alias from an interface
chdev -l en0 -a delalias4=192.168.1.3,255.255.255.0