Generate unique IP/Mac from samd21 serial
You can use a random MAC address, either with a fixed OUI or a completely random address. There’s online tools for generating them, search ‘random MAC address generator’. 

(Maybe not relevant for you, but note that both iOS and Android have moved to using random MAC addresses to thwart tracking of their phones on networks. That’s a different topic.)

If you want your own OUI (vendor ID) you can obtain one from the IEEE for a nominal fee. (Looks like TeensyMAC assigns one so they have you covered there. Use that when you generate the random one with the online tool.)

The IP address is usually handed out by a DHCP server on your network, it’s not pre-assigned in the device. This means you should be implementing a DHCP client so the device can ask for one. TeensyMAC has nothing to do with that, except you need to have an assigned MAC address before you can make a DHCP request so the server can respond to you.

If you don’t want to do DHCP, and instead want to have a fixed IP, then you will need to provide a serial port or some other method to set it when installing the device onto a network. That’s because you don’t get to choose the net/subnet/mask, that’s determined by whoever owns the network. And, IP addresses are not guaranteed to be unique except at the local LAN subnet level.