Since the Mac mini doesn’t have two Ethernet ports, some people are concerned that they can’t have multiple IP addresses on their Mac mini server. Luckily, this isn’t the case.
To have a second ip address, you just need to make a second “Ethernet” entry in the Network Settings.
In System Preferences, choose “Network” and then click on the “+” at the bottom of the left hand column. In the “Interface” drop down, choose “ethernet” and then give it a name. Then, you can configure it with an additional ip address. Now your Mac mini can be accessed using either ip address.