I have a Class C IP address assigned by our ISP.
I need to segregate the networks into 200 hosts on internal lan and 50 hosts on DMZ lan.
Can you let me know, the subnet mask that I need to assign for the internal and DMZ networks
That's correct and to make life easier I would suggest making the DMZ a different network number. For example if you're inside scheme is 192.168.0.0/24, I would make the DMZ 10.1.2.0/24. Keeping a class C subnet (ie not subnetting) will keep everything simple when configuring and troubleshooting.