Subnet mask

An IP address is not complete without being associated with a subnet (work) mask. The subnet mask has the following characteristics and responsibilities on a network:

• IPv4 subnet masks are 32 bits in length, while IPv6 subnet masks are 128 bits.
• A subnet mask is used to identify both the network and host portions of an IP address.
• A subnet mask is used to assist us and network devices in determining the total number of networks, as well as the total usable IP addresses that exist on an IP subnet.
• The subnet mask is used to help a host device determine whether a packet should be sent to the default gateway if the intended destination is beyond the local network.

As we have learned in the previous sections, there are typically three classes (A, B, and C) of assignable IP addresses for both public and private address spaces. Similarly, there are three default subnet masks for each class of IPv4 address.

The following are the three default subnet...