IS the IP range 5.x.x.x a special address? I recall himachi used that range for its VPN as it was a "forbidden" ip address for some reason,
As far as I know, 184.108.40.206/8 is a usuable address space on the internet and isn't reserved for any special ranges. You can check out RFC 5735 to confirm: Here's the link: https://tools.ietf.org/html/rfc5735
The Hamachi guys were smart and resourceful. They needed a cheap and dirty way to create VPN connections on the public internet. They did not want to use RFC-1918 addresses as many routers on the live internet will just drop these packets. Also, they needed a system that for sure would not collide with any of the current IP Address assigned to any machine their users might install on.
History lesson: In the early days of the Internet, class A blocks were handed out all over the place. I know that Don talks about this in some of the shows. Pretty much anyone could get a class A block. Apple computer still owns the entire 220.127.116.11/8 address space to this day, and we gave up the entire 127.0.0.0/8 block just for loopback addresses. (I guess even computers love to hear themselves talk)
The Hamachi guys figured out that the class A network 18.104.22.168/8 had been assigned to someone but was not in use on the public internet -- so they essentially hijacked these addresses for their VPN service. At some point, the IANA pulled this block back and re-allocated it to a company called RIPE. This left Hamachi with a problem that they were going to have IP address conflicts, but they were able to search around and find another block of suitable addresses. It turns out that the 22.214.171.124/8 network was assigned to the British defense department, who does not just hang all of their systems off the public backbone with public IPs. So the 126.96.36.199/8 block is assigned but not in real use on the public internet. Hamachi switched from using 5.x.x.x addresses to using 25.x.x.x addresses a few years ago.
Hamachi also supports IPv6 addresses, which are available in bulk, but the above IPv4 hack is useful for people playing older LAN based games over a Hamachi VPN. If you are a fan of any of the old LAN games (Quake, UT, AOE, etc) then Hamachi is a way that you can play these with your friends without having to do the old LAN party.
Wikipedia of course has an entry on Hamachi, but if you are interested in more deep details then you can check out the Security Now guys at:
They have done a few deep dives into Hamachi in the past.