Whois 172/12

Hi all, Tearing what's left of my hair out. A customer is getting scanned by a host claiming to be "172.0.1.216". I know this is bogus, but I want to go back to the customer with as much authoritative umph as I can (heaven forbid they just take my word). I'm pretty sure I read somewhere once that 172/12 was "reserved" or something like that. All I can find now is that 172/8 is "administered by ARIN". Lots of information on 172.16/12, but not a peep about 172/12. If anybody could provide some insight as to the allocation/non-allocation of this block, it would be much appreciated. Thanks. Ted Fischer