IP and MAC is great for starters. And as far as 100% protection goes, I will consider this project a success if I can detect most of the possible hacks
Start with low expectations, detecting the devices on-line is a good first step.
How would I start to program such an IP scan? I found no similar programs anywhere.
how would you do it if you were the Arduino ;)
you would go through a list of addresses and check if you can connect to them. (e.g. ICMP-ECHO aka PING)
If the device responds to PING you can check all TCP/UDP ports of that device.
If the device does not respond to PING, you still might to do a port scan for that IP as response to PING is not mandatory.
If you get a response you should probably look into the ethernet library where to find the MAC address
This might be a place to start - http://playground.arduino.cc/Code/ICMPPing -