simple DHCPv6 config with /56-Prefix

Hello,

I got 2001:db8:0:100::/56  for my LAN, and what I want is the following:

that the DHCP only hands out IPv6 addresses within this range:

2001:db8:0:17f::/64, my dhcpd6.conf looks like this:

<dhcpd6.conf>
ddns-updates off;
update-static-leases true;
allow leasequery;

authoritative;

default-lease-time 7200;
max-lease-time 86400;

dhcpv6-lease-file-name "/var/lib/dhcpd/dhcpd6.leases";

subnet6 2001:db8:0:100::/56 {
         range6 2001:db8:0:17f:0:0:0:0 2001:db8:0:17f:ffff:ffff:ffff:ffff;
         range6 2001:db8:0:17f::/64 temporary;

         option dhcp6.name-servers 2001:db8:0:100::1, 
2001:db8:0:101::444e:5330;
         option dhcp6.domain-search "example.com";

         option dhcp6.info-refresh-time 600;
         option dhcp6.preference 255;
}
</dhcpd6.conf>

the router - runs BIND, DHCP, DHCPv6, ... - has on its LAN side the 
following IPv6 address
2001:db8:0:100::1/56         <-- is this correct?

I also gave the LAN interface this scope local IPv6 addess: fe80::1/64

I configured my workstation [Win10] with this:
       2001:db8:0:110::1 and for the subnet prefix length I gave 56 and 
fe80::1 as the default gateway

and my local mail server [a Linux] with this:
      2001:db8:0:101::4d:4149:4c30/56

now the strange thing: when I run VM on my workstation, that uses DHCP, 
it gets
e.g. 2001:db8:0:17f:34a8:8ba4:67c7:fbd
(if it is a linux it does show 128 as the prefix length)

why is it possible for this VM to connect to my workstation without any 
problems,
but has troubles connecting to the mail server?

when I change the prefix length of the mail server from 56 to 64, it works?

why is that?

Thanks,
Walter

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