Is 255.255.255.254 a vaild subnet mask?

Answer 1

It's valid and it's called a noncontiguous subnet mask since the 16 bits for

the subnet mask are not contiguous. The RFC's, however, recommend against

using noncontiguous subnet masks.

Answer 2

According to the standard RFC3021 (IETF):

Using a 31-bit prefix length leaves only two numbering possibilities (see Section 2.1), eliminating the use of a directed broadcast to the link (see Section 2.2.1). The limited broadcast MUST be used for all broadcast traffic on a point-to-point link with a 31-bit subnet mask assigned to it.

So it is a valid netmask for point-to-point networks only, but this is an exception: for other netmasks, the first and last address in any range are reserved (network address and broadcast address).

However you can't use 255.255.255.254 as a netmask on Windows machines, since Microsoft guys didn't read the spec (or they considered you cannot do a point-to-point connection with your computer). So on those machines, the highest mask would be 255.255.255.252, using 30 mask bits resulting in 2 usable addresses + one broadcast address + one address representing the network.

Answer 3

Yes. The subnet mask of 255.255.0.0 (/16) is used in the Class B network of 172.16.0.0.