What is the relationship of prefix length and the corresponding subnet mask?

The correct relationship between prefix length and subnet mask is that as the prefix length increases, the subnet mask actually decreases in terms of the number of bits used for the network portion of the address. Prefix length refers to the number of bits in the subnet mask that are set to '1', which defines the network portion of an IP address.

For example, a subnet mask of 255.255.255.0 corresponds to a prefix length of /24, indicating that the first 24 bits are for the network, leaving 8 bits for host addresses. If the prefix length increases to /25, the subnet mask becomes 255.255.255.128, which allocates one more bit for the network and reduces the number of bits available for host addresses.

This relationship implies that as the prefix length goes up, the subnet mask effectively becomes more restrictive in terms of the number of host addresses that can be assigned within that subnet, even though it is often represented in a way that makes it seem like the mask is ‘decreasing’ when simplified in terms of individual bits configured as ones versus zeros.

Understanding this relationship is crucial for effective IP address management in networking, particularly in the design of subnets and allocating address space.