For some reasons, it’s beneficial to have two ethernet ports on a Mac mini. (As opposed to just adding a second ip address to the same ethernet port.) For instance, Leopard Server DNS works best when having 2
physical ethernet ports for using 1 as the server.domain.com and the
other as mail.server.com (using SSL for mail).
Apple released a USB Ethernet Adapter with the MacBook Air since it comes without an ethernet port at all. Luckily, this adapter will also work on a Mac mini. Just plug it into a USB port and it is instantly recognized.