how dhcp works when there are 2 range of IP's are derfined
 

consider that there are 2 range defined in my DHCP conf file.
Then which range IP will be given to a client pc when it request for IP.
if given ranges are of different classes or with different subnets

When you say range do you mean two different ranges on the same subnet like 192.168.0.1-10 and then 192.168.0.45-55 Or do you mean two different subnets like 192.168.1.1-254 192.168.3.1-254? Usually it will hand out IP's sequentially. So if you have it set up like in my first example of different ranges on same subnet then once it has handed out all the 1-10 and the dhcp server has a record of a lease for a given IP in that range it will then start handing out in the 45-55 range.