Hello Rahul
If you were to use three separate subnets to accommodate 600 hosts then you could create them, but they would still be separate subnets. For example, you could use
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
That would give you 256*3 = 768 IP addresses.
However, you would still have three SEPARATE subnets each requiring a network address, a broadcast address and a default gateway. You would also require routing to communicate between the subnets. For example, a host at 192.168.0.26 needs to go through a router to reach 192.168.2.26.
If you want to create ONE large subnet, you must find the smallest subnet that will accommodate at least 600 hosts.
So
a subnet mask of 255.255.255.0 or /24 gives you 255 IPs
a subnet mask of 255.255.254.0 or /23 gives you 512 IPs
a subnet mask of 255.255.252.0 or /22 gives you 1024 IPs
Notice that the sizes of the subnets always double in size so you go from 512 to 1024 addresses. There is no whole subnet that gives you 768 hosts (256*3).
A subnet mask of 255.255.252.0 or /22 which gives you 1024 IPs is equivalent to the merging of FOUR /24 subnets.
I hope this has been helpful!
Laz