There are currently two different ethernet gadgets available; ethernet_ecm and ethernet_rndis (although ethernet is also recognised as an alias to ethernet_ecm).
You have two extra configurations available to you in options.txt if you are using one of the ethernet gadgets.
eth_hostaddr- The MAC address of the host on the "other side" of the ethernet adapter.eth_devaddr- The MAC address of the ethernet adapter itself.
The difference is very slight and is to do with some funkiness on the part of Windows. RNDIS (or Remote Network Driver Interface Specification to it's friends) is a proprietary Microsoft protocol for handling USB-To-Ethernet adapters. If you want to use the USB Ninja with a Windows host, you're going to need to use ethernet_rndis.
While most (if not all!) Linux distros (and MacOS) also support RNDIS, some embedded devices may not. That's where ethernet_ecm comes in! ECM (or Ethernet Control Model) predates RNDIS but is supported by pretty much everything except for Windows. In general, if you aren't targetting Windows, use ethernet_ecm.
Once an ethernet gadget is active, a small DHCP server is started that will assign 10.0.0.2 to the host (you may need to run something like dhclient on Linux depending on your distro). The name of the network interface will also vary depending on your OS, but shouldn't be too hard to figure out. Once you're connected, you should be able to ssh into the Pi with:
ssh [email protected](default password is alarm).
NOTE: The network interface used for the connection on the Pi is usb0.