Example: "Class A IP addresses have a netmask of 255.0.0.0."
A netmask is used to define a range of IP addresses. It is similar to a subnet mask, but is used to define classes of IPs rather than a range of IPs that may be used within a network. For example, IP addresses of class B have a subnet mask of 255.255.0.0. This means the IPs must all have the same first two sections, but may have different numbers for the second two sections.
Because a limited number of IP addresses are available, most IP ranges are assigned as Class C, which has a netmask 255.255.255.0. This defines a range of IPs that have identical numbers in the first three sections, but may contain any number between 0 and 255 in the final section. Therefore, a Class C netmask defines a range of 256 different addresses.