Linux prefix length when obtaining an address via DHCPv6 (IPv6). Why /128?

Question

linux

Good afternoon, colleagues. Already broke his head when setting up a DHCPv6 server. Introductory: I have a PC with Ubuntu 18.04 and an ISC-DHCP server. A simple configuration is raised there, which simply issues an IPv6 address from the pool:

subnet6 2001:db8:100:1::/64 {
range6 2001:db8:100:1::100 2001:db8:100:1::130;
}

The server and client are connected via an L3 switch. So, clients get their IPv6 address from the pool. but always with /128 prefix length. Tried different versions of Linux and Windows. Accordingly, there is no sense from such an address, because. they can't ping anyone.
I know that in IPv6 the default gateway and prefix length are given by local IPv6 routers. I have already raised the RADVD daemon on the server so that it announces the prefix and it does it. I also raised such an announcement on the L3 switch and it will also announce it. But DHCPv6 clients don't care, they still get the /128 address.

Question: what am I doing wrong?

Answer 1

try explicitly specifying the "Mask" option in the DHCP server.
IPv6 is not at hand, on IPv4 this is the subnet-mask parameter

Answer 2

Everything is correct. DHCPv6 only issues an address. The prefix information comes to the RA and is recorded in the host's routing table.

Answer 3

What about RA?
What addresses and prefixes hang on the interface of the router broadcasting RA, what flags (Other, Managed, on-Link)?