In message <20170629150630.glfvte2ures27p2n@Vurt.local>, Job Snijders writes: > On Wed, Jun 28, 2017 at 11:09:25PM +0200, Thomas Bellman wrote: > > On 2017-06-28 17:03, William Herrin wrote: > > > The common recommendations for IPv6 point to point interface numbering > > > are: > > > > > > /64 > > > /124 > > > /126 > > > /127 > > > > I thought the only allowed subnet prefix lengths for IPv6 were /64 and > > /127. RFC 4291 states: > > > > For all unicast addresses, except those that start with the binary > > value 000, Interface IDs are required to be 64 bits long and to be > > constructed in Modified EUI-64 format. > > > > (and addresses starting with 000 are only used for special things, > > like the localhost address ::1). And then RFC 6164 adds /127 to the > > allowed prefix lengths. > > > > I know that many devices allow you to configure any subnet size, but > > is there any RFC allowing you to use e.g. /124 or /126? > > Breaking the law! Some IETFers will come hunt you do, be aware! ;-) > > Here is some historical perspective looking at the IETF standarsd and > current Internet-Drafts: > > RFC 3513 "only /64 is valid" > RFC 3627 "don't use /127, use /126 if you must" > RFC 4291 "reaffirming: only /64 is valid" > RFC 6164 "a /127 is OK to use too" > RFC 6583 "there are problems with /64" > RFC 7421 "/64 is the best!" > RFC 7608 "every prefix length must be forward-able" > RFC 4291bis-07 "fine, /64 and /127 are valid, but nothing else!" > draft-bourbaki-6man-classless-ipv6-00 "IPv6 is classless FFS" > RFC 4291bis-08 "fine, /64 and /127 are valid, and anything defined in future > standards, and anything configured manually" > > Quoting from 4291bis-08: > > """ > Interface Identifiers are 64 bit long except if the first three bits > of the address are 000, or when the addresses are manually > configured, or by exceptions defined in standards track documents. > The rationale for using 64 bit Interface Identifiers can be found in > [RFC7421]. An example of a standards track exception is [RFC6164] > that standardises 127 bit prefixes on inter-router point-to-point > links. > > Note: In the case of manual configuration, the Prefix and > Interface Identifier can be any length as long as they add up to > 128. > """ > source: > https://tools.ietf.org/rfcdiff?url2=draft-ietf-6man-rfc4291bis-08.txt > full file: https://tools.ietf.org/html/draft-ietf-6man-rfc4291bis-08 > > So, what it boils down to: if you want to use SLAAC, you should use a > /64, if you don't need SLAAC, do whatever makes sense for you. And never > be greedy: give your end-users a /48, there is plenty of space to go > around.
And that should apply to cell phones as well. A single /64 from a ISP to a customer is a stop gap assignment. 3GPP supports DHCP-PD it should be enabled in the back ends. > Kind regards, > > Job -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: ma...@isc.org
