See the man page on ifconfig for more info.

(As an example, I'm using hme0 - your interface may be different)

You can set up a logical/virtual interface by setting the IP or a hostname in a /etc/hostname.hme0:1. If you use a hostname, it has to be in /etc/hosts. And you can either reboot or use the ifconfig command to set up the logical/virtual interface.

ifconfig plumb hme0:1 inet 172.16.1.2 netmask 255.255.255.0 broadcast 172.16.1.0 up