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Comment by Scene_Cast2 | Hacker News Reader

Could you give a brief overview of your current understanding? I'm curious as to why /60 would be too limited, and how it's different from IPv4.

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[not OP, but hey…] There’s no need for DHCP with IPv6. Clients choose the last 64 bits of an address randomly. The address space is huge — zero probability of a duplicate address.

The first 64 bits therefore identify the network. If your ISP routes a /60 to you then you get to split that into 16x /64s.

That’s probably fine, but the IETF recommendation is to dish out a /56 to small sites and give them a /48 if they ask for it. ISPs usually have multiple /32s at their disposal.

It’s not about number of available addresses or networks. The joy of IPv6 is the addresses are so wide you can bring back hierarchical addressing. No internal routing chicanery is needed.

If your site has 64 buildings then a /56 lets you assign a /62 to each, with 4x VLANs in each building eg for printers, guest net, phones, lighting, admin, with even a subnet for each business function in the building.

Some way of automatically propagating that network prefix to anything connected to the network would be nice. Otherwise we're just stuck punching in IP addresses like barbarians.

Not sure what you mean, but there's definitely no need to type addresses anywhere.

Router Advertisements propagate the subnet information (prefix, dns server, etc.) to clients in the network. Prefix delegation allows downstream routers to request subnets from upstream routers. MDNS lets network devices announce their services to the subnet.

RA: router advertisements.

The good riddance part of IPv4 DHCP is the server maintained state. The broadcast-config thing is still there. RADVD is the standard Linux daemon for this.

> The address space is huge — zero probability of a duplicate address.

Not necessarily, there could be a flawed SLAAC implementation where the programmer just hard coded the default address after getting the PD. Or other some such silliness.

Yes it breaks spec. But there's enough devices out there that break the link local configuration spec that I would not be surprised one but if someone pushed out a broken implementation of SLAAC on a commercial product.

DAD at the other end kills bad actors like this fast. But if the implementation at both ends is broken…?

The idea is that there's SO much address space -- you should never need to consider a question like: "is /60 to /64 enough for all of my subnets?" when planning your network. Subnets should be created because they make sense from an organizational point of view. The amount of available bits shouldn't be a practical consideration.

Also, remember, NAT is highly discouraged, so you'll (maybe) eat up another subnet if you run something like Minikube on a laptop.

Probably because /64 is the smallest recommended subnet size, and a /60 has “only” 16 /64 subnets within it.

IPv6 has Stateless Address Auto-Configuration (SLAAC) as preferred method for address assignment. This allows clients to generate addresses for themselves as needed. For that to work efficiently, the address space needs to be sufficiently large that collisions are unlikely even in larger networks. Because of that, it mandates a subnet size of /64.

More generally, a big difference in v6 is that you no longer have to plan subnet sizes at all. Whereas previously you'd carefully choose the next available address to minimize address waste, with v6 you can just assign the addresses in whatever way makes sense to you.

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