Internet Sharing, same LAN I'd like to share internet from ethernet (10.0.100.2) to Wi-Fi.

But when I do so and then I connect a device via Wi-Fi I get a whole different network: 192.168.2.0. Is there a way to use the same 10.0.100.0 network? 

I need it because I have internal DNS and need to debug stuff over the LAN.
There is no need to use the same network. Usually you can simply populate your DNS-server with the IPs and the names of the hosts in the 192.168.2.0 network.

If you restricted allowed DNS queries to a particular network (i.e 10.0.100.0/24), you have to extend this to the 192.168.2.0/24 net.

In the network settings of the devices in the 192.168.2.0/24 network you have to set up your internal DNS-server as DNS-server.