A dual node must determine if the peer can support IPv6 or IPv4 in order to check which IP version to use when transmitting. The control of the information that goes in the name service enables a dual node to determine which IP version to use. You define an IPv4 node's IP address and the IPv6 node's IP address in the name service. Thus, a dual node has both addresses in the name service.
The presence of an IPv6 address in the name service also signifies that the node is reachable by using IPv6. However, the node is only reachable by nodes that obtain information from that name service. For example, placing an IPv6 address in NIS implies that the IPv6 host is reachable by using IPv6. However, the IPv6 host is only reachable by IPv6 and dual nodes that belong to that NIS domain. The placement of an IPv6 address in global DNS requires that the node is reachable from the Internet IPv6 backbone. This situation is no different than in IPv4. For example, the mail delivery operation requires that IPv4 addresses exist for nodes that can be reached by using IPv4. The same situation is true for the HTTP proxy operation. When no reachability exists in IPv4, for instance, because of firewalls, the name service must be partitioned into an inside firewall and outside firewall database. Consequently, the IPv4 addresses are visible only where the IPv4 addresses are reachable.
The protocol that is used to access the name service is independent of the type of address that can be retrieved from the name service. This name service support, coupled with dual-stacks, allows a dual node to use IPv4 when communicating with IPv4-only nodes. Also, this name service support allows a dual node to use IPv6 when communicating with IPv6 nodes. However, the destination must be reachable through an IPv6 route.