What is the netmask for a CIDR /16?

A CIDR (Classless Inter-Domain Routing) notation of /16 indicates that the first 16 bits of the IP address are used for the network part, while the remaining bits are used for host addresses within that network. In an IPv4 address, which consists of 32 bits, a /16 means that the network mask has 16 consecutive 1s followed by 16 consecutive 0s.

When we convert this to a decimal subnet mask, the 16 bits of '1' are represented as 255 in the first octet and 255 in the second octet, while the last two octets, filled with '0's, contribute 0 to each of them. Therefore, the netmask in decimal form is 255.255.0.0.

This is why the answer is correct: it accurately reflects the subnet mask associated with a /16 CIDR notation. The other options do not represent a /16 netmask, as they pertain to different CIDR notations with varying distributions of network and host bits.