CIDR writes an IPv4 address followed by the number of leading network bits. In 192.0.2.130/26, the first 26 bits identify the network portion and the remaining six bits select an address inside a 64-address block. The prefix is not a host count and it does not describe whether a device is reachable.

A valid mask is one contiguous boundary

A /26 mask has 26 one-bits followed by six zero-bits: 11111111.11111111.11111111.11000000, or 255.255.255.192. A dotted mask can be converted back to CIDR only when all ones precede all zeroes. A pattern such as 255.0.255.0 is not a subnet mask because the boundary restarts.

Masking finds the start of the block

Bitwise AND between 192.0.2.130 and the /26 mask clears the six host bits, producing network 192.0.2.128. The inverse mask, or wildcard 0.0.0.63, sets the possible host bits. OR-ing it with the network gives the last address 192.0.2.191. This same unsigned arithmetic works for every prefix from /0 through /32.