Hlo,

Put the following command:-

ip address 10.1.2.1 255.255.255.0

The problem is that you are trying to assign the network address on a interface. The 10.1.2.0/24 is a network address.

Thanks

Answer:172.20.225.1

subnet mask is 255.255.255.128 so block size will be 

256-128=128

so  Subnetwork will be 172.20.225.0 and first valid host 172.20.225.1

There are 8 collision domain because

hubs are in one collision domain

switches have per port collision domain