RFC 950 defines the use of a subnet mask (also referred to as an address mask) as a 32-bit value that is used to distinguish the network ID from the host ID in an arbitrary IP address. The bits of the subnet mask are defined as follows:
Each host on a TCP/IP network requires a subnet mask even on a single segment network. Either a default subnet mask, which is used when using class-based network IDs, or a custom subnet mask, which is used when subnetting or supernetting, is configured on each TCP/IP node.
Now come to question we have totally 8+8+8+3 ( 110100 ). i.e 27 1's we have.
now no. of zero =32-27=5
so total no. of ip address possible in this subnet = 25
total no. of host = 32-2=30
=>>>>> read this
but for ipv6 this is not used and rfc made it as a rule