Subnets
A sub network is a prefix of an IP address normally of length 8, 16, or 24. These form the hierarchy of IP addresses.
Subnets can either be represented as an IP address followed by a slash with the prefix length such as 123.145.0.0/16. Or as an IP address with a network mask such as 123.145.0.0 with 255.255.0.0 which represents the same information.