/64 prefix
more would be a waste of address space
The problem is when an ISP for a home connection such as DSL is only giving out a /64 IPv6 prefix, especially for reasons such as "conserving IPv6 address space". giving only a /64 and nothing more is not enough for comfort and unnecessarily restricts your customer. The general consensus in the IPv6 community is that a home customer has to get a shorter prefix.
update:
- It seems tweakDSL, which this annoyance was originally written for, fixed this problem and has been giving out /56's as of 2009-09-17, but i'm keeping this up to apply to any other ISP that may be giving out /64's, or that are planning to do so, and to hopefully convince them to do reconsider this.
- As of 2011, comcast is giving out /64's for their ipv6 pilot but they are receiving plenty of feedback that this is a bad thing, and say they may be giving out more later on.
- recommendation for assigning a /56 to small sites/homes became a RFC.
"with a /64 you can number 2^64 of devices, which is enough"
Thinking with how many bits can number how many devices is a naive way of thinking, the ipv6 address is structured in many ways. Ipv6 address space is not measured in addresses, but in subnets. One subnet is a /64. A subnet is normally a single layer 2 lan without any routing or firewalling inside it.
Structure of an ipv6 address:
| globally routable prefix | customer prefix | subnet prefix | interface address |
| typically up to /32 | typically between /48 and /64 | /64 | 64 bits |
"why does a subnet need a /64?"
RFC 4291: "All Global Unicast addresses other than those that start with binary 000 have a 64-bit interface ID field." 64 bits for the host is specified by design, by convention, and in implementations. stateless autoconfiguration needs a /64 subnet and is more widely supported than stateful autoconfiguration. privacy extensions (rfc3041) needs a /64 subnet. being able to use autoconfiguration is convenient. being unable to use 64 bits for one subnet complicates the situation for the user and may cause problems, as in, it is uncomfortable.
"a home does not need more than one subnet"
You will need atleast 2 if you want both a wired lan, and wireless, and want control over how you set it up with routing, firewalling, public/private subnets, etc, or for experimentation/research.
"if you need those, you want too much for a home, and you need the more expensive business DSL"
Business connections give you, for a high price, access to resources which are really scarce or costly, such as: an ipv4 allocation, 1 on 1 contention, and uptime guarantees. a /56 allocation in ipv6 is not at all scarce like a /29 is in ipv4 so it is no comparison. it is generally agreed on that a home can have a small number of subnets.
"giving a /60 would be a waste. it is a scarce resource. the address space will be used up too fast."
all ISP's have a /32. /32 is the smallest prefix length which is globally routable without problems. With a /32, you can give the following number of prefixes to your end users:
| prefix length | number of allocations within a /32 | number of subnets in allocation | comment |
| /48 | 65,536 | 65,536 | recommended for large sites >100 subnets |
| /52 | 1,048,675 | 4,096 | recommended for small sites <100 subnets |
| /56 | 16,777,216 | 256 | recommended for homes <10 subnets |
| /60 | 268,435,456 | 16 | given out by some ISP's for homes |
| /64 | 4,294,967,296 | 1 | recommended only if one subnet is needed |
conclusion: when giving out /60's, even with millions of customers, typical for large ISP's, you will never use more than, say, 2%, of your /32, that is not using any significant portion of the address space.
"i'm not allowed to ever run out of my /32"
the RIR reserves a /29 for an ISP, even if they give out a /32: this is why the prefix always ends with a 0 or an 8. very large ISP's have been given bigger allocations already (TeliaSonera: 2001:2000::/20; Orange: 2A01:C000::/25)
"i use 6to4rd to give ipv6 to my end users"
6to4rd requires 32 bits to point to the customer within the ISP's prefix. this is used by free.fr/proxad. however, they use a /28, to give /60 to each of their customers. i think 6to4rd is wasteful and is to be avoided.
sources
RFC6177 - IPv6 Address Assignment to End Sites
RIPE policy for allocation to end sites
RFC4291 - ipv6 address design
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