Error: The device '...' does not have a subnet mask.

The subnet mask is a bit mask, consisting of 4 (for IPv4 networks) or 6 octets (IPv6 networks). Usually, the subnet mask is displayed and has to be entered as Integer values (0-255).
The subnet mask describes IP networks and specifies how many bits at the beginning of an IP address are interpreted as net prefix. It separates each IP address into a network prefix used for the network identification and a device part for the device identification. This way, the subnet mask determines which IP addresses a communicating device with a specific IP address has to search in its own network and which addresses are located in other networks.

The network prefix must be identical for all devices within the same network (subnet) and the device part must be different for unique device identification within a subnet.

The determination (i.e., separation) of the network identifier and the device identifier is performed by a bitwise AND operation of the device IP address and the subnet mask.

Rules for a valid subnet mask

To form a valid subnet mask, the following rules apply:

Example (IPv4)

The subnet mask 255.255.255.0 corresponds to the bit sequence
11111111.11111111.11111111.00000000

In this example, the first 24 bits are set to 1 and represent the network prefix. The device part, represented by the last eight 0-bits of the subnet mask, provides a theoretical range of 256 addressable devices. As 0 identifies the network itself and 255 is reserved for the network broadcast, 254 devices can be addressed in a network with this subnet mask.