Iljitsch van Beijnum writes: > When I first learned about IPv6 I felt strongly that 128 bits was too > much, especially since all those bits have to be carried in every IP > packet twice, once as a source address and once as a destination > address. When I first learned about IPv6 I started worrying that it wouldn't last for very long before being exhausted in turn. And I worried a lot more when I saw the mistakes of the past being repeated--the very mistakes that wasted so many IPv4 addresses. > However, since that time I've learned to appreciate > stateless autoconfiguration and the potential usefulness of having > the lower 64 bits of the IPv6 address as a place to carry some > limited security information (see SEND and shim6 HBA). Once it's carrying information, it's no longer just an address, so counting it as pure address space is dangerous. > The trouble is that you need to build in space for growth. You build in space for growth by not trying to allocate address spaces in advance. For example, you allocate 0...0+33 bits for new addresses, and now you've doubled the IPv4 address space (and bought yourself years of additional time), while using up only an infinitesimal portion of the IPv6 space. But when you start chopping the address into sections, you throw almost all the address space away ... and when that happens, you are going to exhaust that space in no time, no matter how many bits it has. Building in space means not allocating it--not even _planning_ to allocate it. Nobody has any idea what the Internet might be like a hundred years from now, so why are so many people hellbent on "planning" for something they can't even imagine? > Unfortunately, at the time IPv6 was created variable length addresses > weren't considered viable. Variable-length addresses are the only permanent solution, unless IP addresses are assigned serially (meaning that all routing information has to be removed). Variable-length addresses work very well for the telephone system, and they'd work just as well for the Internet, if only someone had taken the time to work it out. > The only thing I'm not too happy about is the current one address / > one subnet / /48 trichotomy. Ignoring the single address for a > moment, the choice between one subnet and 65536 isn't a great one, as > many things require a number of subnets that's greater than one, but > not by much. It's a good example of waste that results from short-sightedness. It happened in IPv4, too. > The thing that is good about IPv6 is that once you get yourself a / > 64, you can subdivide it yourself and still have four billion times > the IPv4 address space. It sounds like NAT. > I'm not a huge fan of the HD ratio either, because it substitues a > rule of thumb for actual knowledge. But the point is that EVEN if you > waste 99.9756% in this way we STILL have enough addresses to give > every person living on the planet when the population hits its peak > several /48s which are wasteful in their own right. Famous last words. I've seen virtual memory systems run out of virtual address space, even when that space contained (in theory) more bytes than anyone could ever possibly build into any real-world system. The reason? Careless allocation of the addresses. No matter how many bits you have, you can blow through them in linear time if you allocate them based on bit fields, and it seems that virtually no engineers can resist the urge to do exactly that. > So while I wouldn't want to take away your right to begrudge the way > all of this is done in IPv6, I must object to your conclusion that > we'll run out of IPv6 soon, for any reasonable value of "soon". Well, time will tell, won't it? > I hope good engineers don't think that ... Any engineer setting aside bit spans in an address for future use is thinking exactly that, and he'll be wrong. > Engineers should build stuff that still works reasonably well even if > they get their predictions wrong. Engineers don't like to think that they've left anything out or that they are less than omniscient in assessing what must be done, so many of them are allergic to anything that is simply "reserved for future use." I had the same trouble when I first started in computers, but I grew out of it. _______________________________________________ Ietf@xxxxxxxx https://www1.ietf.org/mailman/listinfo/ietf