While the mac mini only has a single Ethernet interface, it is possible to assign more then one IP address. To achieve this you simple need to create a second Ethernet entry.
Within System Preferences select “Network” and click the “+” button in the bottom left hand corner. Under the “interfaces” drop down, choose “Ethernet” and provide a name (anything will do). This will allow you to configure an additional IP address under the new interface you just created.
To test and make sure everything is working, I recommend running an issuing an ICMP ping from a remote host. You can achieve this under OS X by opening “terminal” and typing “ping xxx.xxx.xxx.xxx”. You can press CNTL+C to end the test.
source: Apple OS X Server Installation Manual