240/4 is tainted. The fact that some code exist somewhere to make it work is good, but the reality is that there are tons of equipment that do not support it.
If you believe that, then don't use it. But don't dictate to me and everyone else what we can and cannot use in our networks. If somebody, somewhere, wants to use 240/4 then they should be allowed to do so without putting additional BUREAUCRATIC roadblocks in their way. IANA's failure to allocate 240/4 to RIRs is a bureaucratic roadblock. ARIN's failure to allocate 240/4 space to THOSE WHO DESIRE IT is a bureaucratic roadblock. IETF's failure to un-reserve 240/4 space is a bureaucratic roadblock. Investigation has shown that the router code and O/S code only requires a very simple change to enable 240/4 to function as normal IPv4 unicast addresses. Vendors have no excuse for not including this change in their next software releases. The impending exhaustion of the IPv4 space is the reason why it is an imperative for vendors to make this change. You might not use it, and I might not use it, but I believe that there will be enough people who can find some use for it that the pressure on the remaining IPv4 space will be diminished. And every extra day that we can buy before IPv4 exhaustion helps people get their IPv6 planning and deployment up to the same "carrier-level" standards as we currently enjoy with IPv4.
Deploying a large network with 240/4 is a problem of the same scale as migrating to IPv6, you need to upgrade code, certify equipment, etc...
Yes we know that, as with any other tecnological change, there is a set of ifs ands and buts that engineers need to deal with in order to use 240/4 addresses. It is good to document what these conditions are so that people don't do something stupid and just treat them as normal IPv4 unicast addresses. But, in general, the people who would request 240/4 addresses are not stupid and will do the right thing.
So, classifying 240/4 as public use is unrealistic now and will remain unrealistic in the near future.
RFC 1918 addresses are not public use yet I will bet that you see them in packets hitting the edge of your network. So you filter them. If you can't handle 240/4 then do the same, just don't tell other CONSENTING networks what they can do.
Classifying it as private use should come with the health warning "use this at your own risk, this stuff can blow up your network". In other words, this is for experimental use only.
This is ridiculous and untrue. There is no evidence that 240/4 addresses will blow up anything. A while back people reported on the NANOG list what happened when they tried to use them. Short answer, nothing happened. That's why vendors need to take out the one line of code that disables these addresses. And the buggy-whip manufacturers like you can just safely ignore the whole business. --Michael Dillon