Virtual interfaces allow a single ethernet interface to listen on additional IP addresses.
Given an ethernet interface hme0 (use ifconfig -a to identify the names of your interfaces), you can create a subinterface called hme0:1 with the following command:
ifconfig hme0:1 plumb
You can set the IP address of the interface to 192.168.1.15 and turn on the interface with the following command:
ifconfig hme0:1 192.168.1.15 up
Unless you do some additional nonstandard things in your network, all of the subinterfaces on a physical interface need to be in the same subnet.
To make the virtual interface persist following a reboot, you can add the ip address or hostame from /etc/hosts in the file /etc/hostname.hme0:1