Since the bits that identify the subnet are specified by a bitmask, they need not be adjacent in the address. However, we recommend that the subnet bits be contiguous and located as the most significant bits of the local address.
Most devices follow this recommendation as far as to enforce it. I have only managed to use non-contiguous subnet masks on Linux-only networks back in 2012; the Windows, OSX, Cisco and HP devices I tested didn't handle/allow it.
