The whole issue of IPv4 depletion and waste seems to be finally getting behind us with the (somewhat) increased deployment of IPv6.

The sole purpose of IPv6 was to solve the issue of IPv4 address space running out. If that is the case then why are governing organisations allocating such large blocks of v6 addresses that are purely and utterly overkill and an obvious waste?

Is there logical reasoning behind the allocations or is it more of a case of a "I'm rich, Lets share them all around!" sort of thing?

For example, I was recently allocated a /48 block of v6 addresses with a single server. That's a staggering 1,208,925,819,614,629,174,706,176 addresses for my single server. I doubt the kernel would let me allocate that many addresses to an interface and I doubt any available NIC would support even a 10000th of them. Why are IPv6 addresses being handed out in such large blocks?

  • 6
    Simply because there are a ridiculous amount of addresses available in IPv6. If a trillion IPs were used every second it would take well over a trillion years to get anywhere near the limit. Check out: The Sheer Size of IPv6
    – user143703
    Apr 12, 2014 at 8:30
  • 4
    So that we never, ever run out, in any foreseeable timeframe, and never again have to use NAT. Apr 12, 2014 at 17:25
  • 3
    IPv6 has a ridiculous amount of possible addresses, but if we scatter /48's around with wild abandon, we'll blow through them uncomfortably fast. Put it this way: IPv6 supports only 65,536 times as many /48's as IPv4 has single addresses, and we've run those out in just a few decades. Apr 13, 2014 at 6:24
  • 1
    @GordonDavisson Yep. Better start address conservation measures now, so that the address pool will last for a few more centuries. Apr 13, 2014 at 16:20
  • 6
    @GordonDavisson I'm not sure you've yet grasped just how big this address space is. You could give a /48 to every man, woman and child on earth and have only used up most of a single /15 worth of space, or 0.003% of the entire IPv6 space. (I assume a population of less than 8 billion souls.) It truly is not nearly as significant a concern as it was with IPv4. Apr 13, 2014 at 16:46

3 Answers 3


The main reason is that stateless address autoconfiguration as per RFC4862 requires a /64 network to work properly. Add to that the assumption that one will want more than a single subnet at one's installation and the difficulty of routing arbitrary multiples of a /64, and the automatic tendency seems to be to assign a /56, or if lazy, a /48.

Oddly, I'm already seeing the first signs of parsimony in the UK. I've had v6 in my home office for a couple of years, now, but recently changed provider. The old one gave me a /56 automatically; the new one gave me a /64, but when I mentioned that I was subnetting happily upgraded me to a /56 without charge.

My guess is that the base allocation will stabilise at a /64 once v6 becomes common, with anyone who has a half-decent reason for it getting a /56.

  • 2
    Having to ask for more than a /64 is not smart of an ISP. Many setups need more than one /64 (e.g. home LAN and guest LAN) and (a) it will cost them money if their support desk has to handle these requests and (b) most users will not know what to ask so (c) devices will start hacking around this with NAT etc. While the ISP has all the IPv6 addresses they need for free, up to giving each customer a /48. Apr 12, 2014 at 11:02
  • 5
    Also, using stateless autoconfig and security extensions, you get a fairly spread out use of address space, so it is no longer economic to scan for machines on the Internet, which is a large attack vector for worms. Apr 12, 2014 at 12:01
  • 1
    At my house I have five physical subnets and a varying number of virtual ones as I do various stuff with VMs and groups of VMs. I also have a /48. The key here is that with IPv6, we no longer think in terms of individual addresses, but in terms of subnets. IPv6 address allocation is meant to provide enough subnets to last the requester for decades. See RFC 6177. Apr 12, 2014 at 17:35
  • I agree with a lot of what you say. That said, it's not like there was much push-back from my ISP. Many people really won't need more than a /64, as they wouldn't know a subnet if it stole their breakfast; for the clueful rest, a quick "do you need a /56" can be a checkbox on an online signup form. The "decades" bit is probably not relevant in terms of home setups, as it's all PA-space and non-portable; most of us don't keep the same ISP for 50 years.
    – MadHatter
    Apr 12, 2014 at 18:02
  • I find a /56 useful at home (eg OpenVPN with routed IPv6; Wi-Fi guest network). But I'll probably only use 2 or 3 /64s out of that at most. A /60 would do me fine (is there such a thing?). Handing a /48 to home users seems like extreme overallocation. Feb 22, 2020 at 21:54

I imagine that routing smaller blocks creates problems for BGP routing - the more smaller blocks, the more routing ALL routers which don't carry a default route need to carry.

Also, while the driving force behind IPV6 is increased address space, IPv6 has a lot of advantages over IPv4. (More efficient routing, simplified network configuration, no more requirement for NAT - if you call that an advantage, better security - IPSec is baked into it)

My impression (and its nothing more then that, although I am on the fringe of the ISP community) is that there is no point in scrounding IPv4 addresses as it will only delay the inevitable - sooner or later the Internet is going to need to IPv6, no point in prolonging the agony by stretching IPv4 further then it has to be. Those who need to invest in upgrading infrastructure will hit the same walls regardless - they may as well hit them now.

  • 1
    I agree with you on the issue with BGP routes, however there comes a point where old tech needs to update to accommodate other new tech. We are basically saying "We've invented this awesome new addressing standard but it's impractical for X, Y and Z existing protocols, let's just waste a few trillion coz we too lazy to do 'nything about it!" Apr 12, 2014 at 8:26
  • 1
    @jduncanator I think thats an inaccurate characterisation. IPv6 can run on top of and alongside IPv4 to ease the migration. Its also not a sudden change - its been ready for deployment since somewhere between 1999 and 2008 depending on your POV. In fact, in the early 2000's it was expected to be fully deployed by 2007, so there has been plenty of time to adopt. All modern OS's support it out the box - that new routers often don't is an indication of lazyness in the industry. (Indeed techos have stretched IPv4 way farther then they should have, including carrier grade nat and SNI)
    – davidgo
    Apr 12, 2014 at 8:34
  • I agree on the less-than-satisfactory up taking of IPv6. I live in Australia where none of the major ISPs provide IPv6 support even tho I have a router that does. Add on top of that the crappy connection to any of the popular v6 tunnels that make it unusable and you can see why people see no "rush" (or even any motivation) to move to IPv6. Something needs to change because at the end of the day, if IPv4 space completely runs out, the consumer ISPs aren't the ones who are going to be affected so there will be no pressure to implement IPv6. Apr 12, 2014 at 8:47
  • 2
    @jduncanator Your country's politiicans are actively hostile to the Internet, so I'm not too surprised. As for NAT, yes, getting rid of it is a significant advantage. It means all the protocols that broke or needed horrifying workarounds to function can now work normally. Which, at this point, is a very large number of them. Apr 12, 2014 at 17:28
  • @JohannesKuhn What does that have to do with anything?
    – Navin
    Aug 30, 2016 at 20:29

I feel like this was generally answered sufficiently (there are 240 trillion /48 allocations, which means every human on earth can receive 30,0000 /48 allocations and we still won't be out). But I will note that 2011's RFC 6177 changed the recommendation for ISPs and RIRs from "provide customer sites a minimum of /48" to "provide customer sites something shorter than a /64, probably a /56, but use judgement"

To quote the RFC:

While the /48 recommendation does simplify address space management for end sites, it has also been widely criticized as being wasteful.

I would disagree with this. Again, there are 240 trillion /48 allocations. Human extinction will proceed us running out. /48s offer way more address space than most sites need, but it's not really wasteful. It continues:

At the same time, it might be tempting to give home sites a single /64, since that is already significantly more address space compared with today's IPv4 practice. However, this precludes the expectation that even home sites will grow to support multiple subnets going forward. Hence, it is strongly intended that even home sites be given multiple subnets worth of space, by default. Hence, this document still recommends giving home sites significantly more than a single /64, but does not recommend that every home site be given a /48 either.


A key principle for address management is that end sites always be able to obtain a reasonable amount of address space for their actual and planned usage, and over time ranges specified in years rather than just months. In practice, that means at least one /64, and in most cases significantly more. One particular situation that must be avoided is having an end site feel compelled to use IPv6-to-IPv6 Network Address Translation or other burdensome address conservation techniques because it could not get sufficient address space.

The RFC also recommends only breaking up allocations on even nibbles, so /60, /56, /52, /48, etc. A /60 provides end users with up to 16 subnets, which is ok, but way less than the 255 subnets private addressing on IPv4 allows. It's not hard to imagine a home user needing more than 16 subnets. Most won't, but it's not hard to imagine.

  • assigning a longer prefix to an end site, compared with the existing prefixes the end site already has assigned to it, is likely to increase operational costs and complexity for the end site, with insufficient benefit to anyone.

I've seen some ISPs are finally getting around to deploying IPv6 for home users, but they're only providing /64 and they aren't providing static prefixes. This means home users can't run more than 1 subnet on IPv6, and that's distressing. It's fairly common for homes to have 1 subnet for most devices and 1 subnet for Guest Wifi. I'd encourage another subnet for IoT smarthome devices since those things seem to have so many firmware bugs that you hardly want them to be able to access the internet, but certainly don't wan them with access to your LAN. With only a /64, a home user would have to either: pick which subnet is IPv6 capable and use IPv4 + NAT for the other subnets or use IPv6 - IPv6 NAT.

I feel like a /128 is reasonable for a single server in some instances, and a /64 in others. But a /64 is never reasonable for a site, and while RFC6177 gives ISPs more leeway, we probably could have stuck with the "always give at least a /48 to end user sites" from 2001's RFC 3177 without harm.

  • The ISPs will not advertise a prefix longer than /48 because even with /48 prefixes, we still have far too many possible prefixes to actually work in a global routing table.
    – Ron Maupin
    Oct 5, 2018 at 3:25
  • My ISP already issues longer than a /48. In fact, they only issue /64 which is too long to for me to subnet without violating a bunch of other IPv6 RFCs. I think you meant they won't issue shorter than /48 prefixes, which is fine. A /48 allows 65536 /64 subnets and very few (if any) organizations need more than that.
    – bobpaul
    Oct 5, 2018 at 19:09
  • That's not what I said. ISPs will not advertise (in BGP to other ISPs and customers) any IPv6 prefix longer than /48. For IPv4, ISPs will not advertise any prefix longer than /24.
    – Ron Maupin
    Oct 5, 2018 at 23:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.