What subnet mask is considered the serial mask?

The subnet mask considered the serial mask is 255.255.255.252. This specific mask allows for a minimal number of usable IP addresses within the subnet, which is ideal for point-to-point links, such as those found in serial connections.

When using a subnet mask of 255.255.255.252, there are only 2 usable IP addresses available in the subnet (one for each end of the serial link), since the subnet mask allocates 2 bits for host addresses from the total of 4 bits remaining in the last octet. This configuration is efficient for serial interfaces that typically connect only two devices, like routers, allowing them to communicate directly without wasting IP addresses on potential additional hosts that won't be used.

The other subnet masks, such as 255.255.255.255, allow for a single IP address, which is typically reserved for use as a host route; 255.255.255.0 provides a larger subnet suitable for local area networks with up to 256 addresses; and 255.255.255.128 allows for 126 usable addresses, making them less ideal for point-to-point serial connections where efficiency and minimal use of IP addresses are crucial.