On Sep 26, 2011, at 6:21 PM, Christian Huitema wrote:
Not exactly to play devil's advocate here, but I don't think these are quite like site-locals. It seems like they're more like "ISP locals". One of the problems with site locals was that they were ambiguous addresses AND that they would need to be used by applications, and we had plenty of experience that said that RFC 1918 addresses caused harm. If 240/4 wouldn't be used or seen by applications at all, the fact that those addresses would be reused in other networks wouldn't be such an issue. Though I agree that these will leak, even if they're never used as application endpoints. If one of them ever appears as a source address on an ICMP reply, for instance, that actually will cause problems - perhaps not problems that directly affect applications, but problems nonetheless. (Then again, tunnel use is now quite widespread, which means that packets travel paths that are completely invisible to the endpoints and look like a single hop to them. And there are of course problems with those, but we sort of deal with them.) Another of the problems with site locals was that there was no clear boundary that corresponded to a site, so why have a special class of addresses for a site? If that's also true for an ISP, maybe an ISP local address isn't such a good idea either. What happens if two ISPs that are each using 240/4, merge? Probably the same kind of mess you get when two enterprises that use RFC 1918 addresses merge. Granted, ISPs might not merge as often as enterprises, but still... It was especially important to get rid of site locals in IPv6 because IPv6 was in very early stages of deployment, and any errors in its design would be magnified over time. By contrast, IPv4 is a dinosaur struggling to take its last unassisted breaths, and which is starting to be put on life support. Some sort of extraordinary measures to keep IPv4 vlable for a short time might be in order, even if those measures would never make sense in IPv6. So my take is that using 240/4 is not an absolute no. But that bit of address space is a very precious resource and there needs to be strong justification for using it, along with reasonable assurance that it will not do significant harm in relation to the amount of benefit it will likely provide. And merely prolonging the life of IPv4 is probably not sufficient justification. Keith |
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