Where do you derive that information from? Right now some of the networks still belong to IANA (--> unused), but how can a system administrator be sure that the network won't be opened within the next few days? In worst case I'd deny access to my network for 254^3 IPs. If you enable logging and use logcheck or something similar, you will notice connection attempts anyway. If you can traceroute them back and find it at the below given URL, open up the net. updates concerning reserved IANA netblocks can be found on http://www.iana.org/assignments/ipv4-address-space . Take a look at it every month or so...
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