A network broadcast uses the largest allowed IP within a sub-net (a network address space). Some examples:
The private Class A network 10.0.0.0/8: the broadcast address for the whole block is 10.255.255.255.
The private 16 x Class B networks 172.16.0.0/12: the broadcast address for the whole block is 172.31.255.255.
The private 256 x Class C networks 192.168.0.0/16: the broadcast address for the whole block is 192.168.255.255.
Normally, a smaller /24 subnet within the above blocks is used for a private network. The following utility can be used to obtain the range of addresses for any CIDR, and to convert between all addressing types (CIDR, Range & Mask) (opens in a new tab):