digraph G {
    node [ shape=box ]
    #rankdir=LR;

    clients [ label="Clients" ]

    subgraph cluster_internet {
             label="Internet"
             dns1 [ label="DNS Server\nPublic IPv6\nPublic IPv4" ]
             dns2 [ label="DNS Server\nPublic Address\nPrivate Address\n(v6 or v4)" ]
    }

    subgraph cluster_v6only {
             dnsv6 [ label="DNS Server\nIPv6 only" ]
    }

    subgraph cluster_private {
             dnspriv [ label="DNS Server\nPrivate IPv4" ]

    }

    dns1->dnsv6 [ label="Forward DNS query via IPv6" ]
    dns2->dnspriv [ label="Forward DNS query via private IPv4" ]

    clients->dns2 [ label="Query via IPv6/IPv4" ]
    clients->dns1 [ label="Query via IPv4" ]
    clients->dnsv6 [ label="Query via IPv6" ]

}