3 ms·

Correct me if i'm wrong, but doesn't an IPv6 /64 give you 2^64 globally routeable addresses? How is that not sufficient?

by mappu 12y ago

simoncion 12y ago
You can't subnet a /64. This RFC [0][1] describes the IETF's thoughts on why allocating a block larger than /64 is to be strongly encouraged. [0] https://tools.ietf.org/html/rfc6177 https://tools.ietf.org/html/rfc6177 [1] Particularly, see page 3
matteotom 12y ago
AFAIK, it's customary to assign a /64 to each machine, and the last 64 bits are used for other purposes (usually the MAC address I think?). https://en.wikipedia.org/wiki/IPv6_subnetting_reference https://en.wikipedia.org/wiki/IPv6_subnetting_reference
- simoncion 12y ago
  Perhaps we're using different terminology, but it's absolutely not customary to assign a /64 to each machine. IME one advertises one or more /64's on a given network, then (either through DHCPv6, or SLAAC [or both]) machines attached to that network acquire addresses within those spaces. These machines will have a XXXX::XXXX/64 address, but that just means that they're attached to a network that advertises a /64, not that that machine has been assigned an entire /64 for its use.
- ghshephard 12y ago
  With IPv6 you assign a /64 to the network. All networks in IPv6 are 64 bits long, with 64 bits assigned to the host's interface ID. There are lots of ways for a host to generate an Interface ID - (http://technet.microsoft.com/en-us/library/cc736439(v=ws.10).aspx http://technet.microsoft.com/en-us/library/cc736439(v=ws.10)...) is a nice reference.
zAy0LfpBZLC8mAC 12y ago
Yes, it does, but individual IPv6 addresses are not directly comparable to individual IPv4 addresses, because protocol mechanisms assume that every network ("LAN") segment has its own /64. For example, IPv6 stateless auto configuration (for automatic stateless address assignment, another central innovation of IPv6) only works with a /64, so you can't use it if you have to subnet a /64. As far as the RFCs are concerned, a /64 is the minimal allocation that is used for a network segment, anything else is a hack.
ghshephard 12y ago
IPv6 doesn't work like IPv4. In IPv4 Parlance, you would be given a CIDR block which you could then subnet as you saw fit. I.E. The ISP might give you a /19, which you could then turn into 32 /24s (or 64 /25s, etc...) With IPv6, /64s are the basic unit, and you don't further subnet the network. So, instead of thinking of "addressable space", you should now think of "deployable networks." . An ISP that hands out a /64 is basically just giving the customer a single network. The smallest network any ISP should hand out to a customer is a /48, which lets the customer deploy up to 2^16, or 65K networks as they see fit.

Correct me if i'm wrong, but doesn't an IPv6 /64 give you 2^64 globally routeable addresses? How is that not sufficient?

by mappu 12y ago

Correct me if i'm wrong, but doesn't an IPv6 /64 give you 2^64 globally routeable addresses? How is that not sufficient?

simoncion 12y ago

You can't subnet a /64. This RFC [0][1] describes the IETF's thoughts on why allocating a block larger than /64 is to be strongly encouraged. [0] https://tools.ietf.org/html/rfc6177 https://tools.ietf.org/html/rfc6177 [1] Particularly, see page 3

matteotom 12y ago

AFAIK, it's customary to assign a /64 to each machine, and the last 64 bits are used for other purposes (usually the MAC address I think?). https://en.wikipedia.org/wiki/IPv6_subnetting_reference https://en.wikipedia.org/wiki/IPv6_subnetting_reference

simoncion 12y ago
Perhaps we're using different terminology, but it's absolutely not customary to assign a /64 to each machine. IME one advertises one or more /64's on a given network, then (either through DHCPv6, or SLAAC [or both]) machines attached to that network acquire addresses within those spaces. These machines will have a XXXX::XXXX/64 address, but that just means that they're attached to a network that advertises a /64, not that that machine has been assigned an entire /64 for its use.
ghshephard 12y ago
With IPv6 you assign a /64 to the network. All networks in IPv6 are 64 bits long, with 64 bits assigned to the host's interface ID. There are lots of ways for a host to generate an Interface ID - (http://technet.microsoft.com/en-us/library/cc736439(v=ws.10).aspx http://technet.microsoft.com/en-us/library/cc736439(v=ws.10)...) is a nice reference.

zAy0LfpBZLC8mAC 12y ago

Yes, it does, but individual IPv6 addresses are not directly comparable to individual IPv4 addresses, because protocol mechanisms assume that every network ("LAN") segment has its own /64. For example, IPv6 stateless auto configuration (for automatic stateless address assignment, another central innovation of IPv6) only works with a /64, so you can't use it if you have to subnet a /64. As far as the RFCs are concerned, a /64 is the minimal allocation that is used for a network segment, anything else is a hack.

ghshephard 12y ago

IPv6 doesn't work like IPv4. In IPv4 Parlance, you would be given a CIDR block which you could then subnet as you saw fit. I.E. The ISP might give you a /19, which you could then turn into 32 /24s (or 64 /25s, etc...) With IPv6, /64s are the basic unit, and you don't further subnet the network. So, instead of thinking of "addressable space", you should now think of "deployable networks." . An ISP that hands out a /64 is basically just giving the customer a single network. The smallest network any ISP should hand out to a customer is a /48, which lets the customer deploy up to 2^16, or 65K networks as they see fit.