IPv6 - Are the last 16 bits of the network ID always reserved for subnetting?

sam

5/10/24, 4:20 PM

I was reading about IPv6 address structure and it has two portions one for the network (Network portion/Network ID) and another for host (Interface ID/Node Part) something like IPv4 has regardless of the size difference.

Most of the references stated that the address is divided into 64 bits for the network (/64 prefix) and 64 bits for the Host , and we can use the last 16 bits of the network part for subnetting .

So is this a rule in IPv6 that the address is always split in half ? and if we want to subnet the address the ISP would give an address with /48 prefix bits leaving only the last 16 bits of the network part for subnetting ? or just a recommended path to follow ?

Regards

1 + 1

ipv6

subnet

Ron Maupin

5/12/24, 7:48 PM

Basically, if you are assigned a `/48` for a site, then you have 16 bits for subnetting (65,536 possible standard IPv6 `/64` networks). It is possible you may get a smaller prefix, e.g. `/32` or even smaller, and that would give you more bits for subnetting. You cannot advertise any IPv6 prefix larger than `/48` on the public Internet, so prefixes advertised on the public Internet will have at least 16 bits for subnetting.

Score:2

Server

Peter Zhabin

5/10/24, 5:01 PM

This is a recommended setup - nothing prevents you from subnetting /64 further, but SLAAC will not work and you'll need to stick to DHCPv6 or static address assignment.

The same holds for /48 and /56 customer assignments, these are numbers defined in allocation guidelines, but operators sometimes have their own bright ideas, e.g. assigning the whole rack one /64..

IPv6 address allocation strategy has been designed with these best practices in mind, resources assigned are abundant for any reasonable use case, so one should stick to them and not reinvent the wheel.

+ 3

sam

5/10/24, 7:03 PM

thanks for the answer , just one last thing , why Stateless Address Autoconfiguration wouldn't work ? isn't the link-local address always /64 ? i mean the interface id must always be 64 and that's make sense ,but if subnetting is used for unique local/global addresses how does this affect link local ? and forgive my poor knowledge .

Peter Zhabin

5/11/24, 12:53 AM

Link-local fe80::/64 addresses would still be assigned as usual, but SLAAC is about assigning globally-unique IPv6 addresses, and for that to work host needs to use it's generated EUI-64 host part together with network address received from the router in RA. Naturally, if your router advertises a prefix longer than /64, there's no way for host to fit EUI-64 into it..

sam

5/11/24, 1:15 AM

thanks a lot for the clarification

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: IPv6 - Are the last 16 bits of the network ID always reserved for subnetting?

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