Subnetting divides a network by borrowing host bits for the network part.
1Formulas
Mask /n has n leading 1-bits
Usable hosts = 2^(32 − n) − 2
Number of subnets when borrowing b bits = 2^b
Network address = IP AND mask; broadcast = network with all host bits 1
Example
192.168.10.77/26: block size 64, so the network is 192.168.10.64 and the broadcast 192.168.10.127; 62 usable hosts.