The group of those addresses is a little bit of a holdover from the outdated classful addressing system used earlier than 1993. Within the classful system, the primary few bits of the IP handle outlined the community measurement. Particularly:
- An handle that began with a 0 bit indicated a “class A” community with an 8-bit prefix (what we would now name a “/8”), so 0.x.x.x – 127.x.x.x had been the category A networks (though 0.x.x.x and 127.x.x.x had been reserved).
- An handle that began “10” in binary indicated a “class B” community with a 16-bit prefix (“/16”), so 128.0.x.x – 191.255.x.x had been the category B nets.
- An handle that began with “110” in binary indicated a “class C” community with a 24-bit prefix (“/24”), so 192.0.0.x – 223.255.255.x had been the category C nets.
There have been additionally class D (multicast) and E (reserved), however they don’t seem to be vital right here.
After they picked handle ranges for personal use, they wished some from every class (to present organizations some flexibility in establishing their inside networks). There weren’t many class A nets, so the simply allotted one: 10.x.x.x. Class B’s had been a bit extra plentiful, so that they allotted a block of 16: 172.16.x.x – 172.31.x.x. There have been loads of class C’s, so that they grabbed 256 of them: 192.168.0.x – 192.168.255.x.
10.x.x.x occurred to be obtainable as a result of it was initially allotted to the ARPAnet spine, and that had been shut down. The opposite two had been merely the bottom large-enough unallocated blocks at school B and C house on the time RFC1918 was written (ref: Daniel Karrenberg on the NANOG mailing listing).
The classful system was rigid and inefficient, so it obtained changed by the present classless system, the place community prefixes can have any size. Below the brand new system, the personal class A web stays the identical (however will get written otherwise: 10.0.0.0/8), and the teams of personal class B and C nets get coalesced into regular blocks of addresses: 172.16.0.0/12 and 192.168.0.0/16. And since we’re classless now, you possibly can break these blocks up any method you occur to search out handy.