Understanding Dual Stacking of IPv4 and IPv6 Unicast Addresses

Increasingly, the public side of network address translation (NAT) devices is IPv6 rather than IPv4. Service providers cannot continue giving customers globally routable IPv4 addresses, they cannot get new globally routable IPv4 addresses for expanding their own networks, and yet they must continue to serve both IPv4 customers and new customers, all of whom are primarily trying to reach IPv4 destinations.

IPv4 and IPv6 must coexist for some number of years, and their coexistence must be transparent to end users. If an IPv4-to-IPv6 transition is successful, end users should not even notice it.

A dual-stack device is a device with network interfaces that can originate and understand both IPv4 and IPv6 packets.

Other strategies, such as manually or dynamically configured tunnels and translation devices exist, but dual stacking is often the preferable solution in many scenarios. The dual-stacked device can interoperate equally with IPv4 devices, IPv6 devices, and other dual-stacked devices. When both devices are dual stacked, the two devices agree on which IP version to use.

The transition is driven by DNS. If a dual-stacked device queries the name of a destination and DNS gives it an IPv4 address (a DNS A Record), it sends IPv4 packets. If DNS responds with an IPv6 address (a DNS AAAA Record), it sends IPv6 packets.

Keep in mind that if you are going to dual stack all of your network devices, the interfaces need both an IPv6 and an IPv4 address. This raises the issue that the Internet has run out of IPv4 addresses, which is the main reason IPv6 is needed in the first place. If you do not have an abundant supply of IPv4 addresses to apply to your devices, you can still use dual stacking, but you will need to conserve your supply of IPv4 addresses by using network address translation (NAT). Building dual-stacked networks with a mix of global IPv6 addresses and NAT-ed IPv4 addresses is quite feasible. Some specific solutions include carrier-grade NAT (CGN), NAT44(4), NAT64, NAT464, and dual-stack lite.

Table 1 describes at a high level how to pick a network addressing technique. In reality, a complete solution might include a set of techniques to satisfy multiple service needs. It is important to understand the backbone technology being used on the network and also to know if the provider has control over the access customer premises equipment (CPE).