It has nothing to do with amounts of addresses, and everything to do with making dividing stuff up for routing easier.
A large ISP entity like comcast or AT&T can now have say a single /16 or /24 allocation and pretty much no matter how much they subdivide up their regional routing, routing to AT&T can easily be coalesced and summarized , and every end customer can still get a /64 till pretty much the end of time.
I totally understand why Comcast, AT&T, Verizon and other service providers would want /16s. They are continent wide providers with millions of customs (millions of sites).
I'm trying to grok why MIT went for a /24 instead of a /32. Because they could?
With their IPv4 /8 they had 2^24 IPv4 addresses to work with, or, from a network perspective, 2^16 /24 (65K) networks, each network containing no more than 254 hosts.
If they had requested a boring /48 IPv6 allocation (that anybody can have just by asking) - they would have had 2^16 /64 networks, and each network could have had basically an infinite number of hosts.
But, this is the IPv6 world, so I would have expected MIT to claim they were a LIR (Local Internet Registry - equivalent of a small ISP or larger) - and asked for a /32 - which would have given them 2^32 networks - or 4 Billion networks to work with. They probably would have assigned the networks by segmenting them on a per site basis - so each site would have had a /48 assigned, so they could have up to 65K sites, each site having 65K networks, each network having (effectively) infinite number of hosts. That is, a /32 would have been far, far, far larger than their /8 was. Easier to manage as well (no VLSM - nothing ever smaller than a /64) And, keep in mind, with a single, no contest request, they could have gotten the /32 adjacent to theirs (another 4 Billion networks, or 65K /48s) so they could aggregate on a /31.
Instead, they've asked for a /24. And I'm just darn intrigued as to why they think they can make use of such an address space. If they weren't constrained with their /8, then a /32 would have been far more than they ever required. (And odds are a /48 would have been sufficient with even modest address management).
I mean, I work with really large mesh networks, millions of nodes, some of our subnets have 20K nodes each on them - we roll out /48s like they are nothing, and even after deploying a couple hundred customers over 10 years, and 25 million nodes, I think we've used up maybe 1500 /48s.
BTW - this doesn't even take into account that they can use RFC 4193 up the wazoo for all sorts of interesting non-globally routable experimental internal networks.
I"m just hoping someone from MIT is reading HN and will clue us in.
That's true when we are talking about /48s. There are enough /48s to hand out to everyone (and then everyone gets 65K networks to work with). In the current Global Unicast space (prefixes starting with 001) there are 2^45 /48s. Effectively unlimited number, as there are 2^33 people on earth - so everyone would get 2^12 /48s.
There are 2^21 /24s in the existing Global Unicast Space. About 2 million. A large number, to be sure. But if every entity in the world MITs size asked for a /24, we'd start burning through them pretty quickly....
Orders of Magnitude are completely different, of course. When MIT got it's /8, (one of just 255 available), it gave them the ability to create 2^16 networks with just 254 hosts on each one.
Now, anyone who wants to can ask for a /48, and they are automatically given it, and they are able to create 2^16 networks, each network with an effectively infinite number of hosts on it. And, there are so many /48s on the (current) Global Unicast space (prefixes starting with 001), that everyone on earth could be granted 4000 of them. And, in reality, ISPs started already conserving, and handing out /56s instead of /48s to most consumers/SoHo - further increasing that number by 255x.
And that's just the 001 prefix. Based on what we learn about allocating space there, new policies can be device for the other 6 prefixes available. We aren't going to run out of IPv6 address space.
The difference is that very, very few organizations require blocks that enormous in IPv6. And the ones that do will most likely never need to ask for more. (For comparison, Comcast, the largest home ISP in the US, has a /20.) But in IPv4, a /16 (or smaller blocks adding up to one) is nothing for even a mid-sized ISP.
Also, the allocation policies are much different. In IPv4, allocations are made to being as small as possible (while still meeting immediate needs) in order to conserve addresses. The cost is that the routing table grows like crazy, since network operators need to keep coming back year after year (or sooner) to get more address blocks.
In IPv6, allocations are made to cover the network operator's long-term needs so that they won't have to get a second, non-contiguous block. By "wasting" address space you cut down on routing table size, since even very large networks only need to announce a small number of routes.
Even with this "waste", we've only allocated a tiny amount of IPv6 space. None of the RIRs have had to go back to IANA for new IPv6 space since 2006[1], even with allocations ramping up in the last few years. (And IANA only gives the RIRs a /12 at a time.) We seem to be fine for the foreseeable future.
But even if somehow we do start using up too much v6 address space, the good news is we can always change allocation practices. Let's say in 2040 we realize we've used up way more of the IPv6 space than we expected we would by then. The RIRs can always change their policies, favoring conservation over smaller routing tables like they used to with IPv4. I seriously doubt that will happen within any timeframe we can reasonably make predictions about, but if it does, we'd have a lot more flexibility than we did with IPv4 depletion.
> and every end customer can still get a /64 till pretty much the end of time.
on Telekom Germany every customer get's a /56
Fun fact their router's (Speedport) can't do prefix delegation to other routers. you need to buy one from another vendor
A large ISP entity like comcast or AT&T can now have say a single /16 or /24 allocation and pretty much no matter how much they subdivide up their regional routing, routing to AT&T can easily be coalesced and summarized , and every end customer can still get a /64 till pretty much the end of time.