Subnet mask

For every IP address, there's an accompanying subnet (work) mask. This address is used to define the following:

• The subnet mask is used to indicate the network and host portion of an IP address
• The subnet mask is used to determine the number of available IP addresses on a network
• If two or more devices, such as computers, are communicating over multiple networks, the subnet mask of each device determines whether a computer should send the packet to the default gateway/router

There are three default classes of the subnet mask:

• Class A: 255.0.0.0
• Class B: 255.255.0.0
• Class C: 255.255.255.0

Each IPv4 address class has their own default subnet mask. An IP address such as 10.10.10.1, which is a private Class A address, will use the subnet mask of 255.0.0.0. Another example we can use is the address 191.5.4.6, which is a public Class B address. This address will...