When a device is configured to use both IPv4 and IPv6 at the same time, it does not “choose” one of the protocols as the “default gateway” in the traditional sense.

Instead, it uses both protocols independently, depending on the communication and the destination you are trying to connect to.

The choice between using an IPv4 or IPv6 route to send packets depends on several factors, including system configuration, route availability, and the preferences of the operating system or application in question.

Here are some key points to understand how this works:

1. Dual-Stack Configuration

The most common configuration on devices that support both IPv4 and IPv6 is known as “dual-stack”. In this configuration, the device maintains independent protocol stacks for IPv4 and IPv6, allowing it to operate with both types of addresses simultaneously. Each stack has its own set of configurations, including its own default gateway.

2. Protocol Selection

The decision whether to use IPv4 or IPv6 for a specific connection is usually based on name resolution and route availability:

• If a hostname resolves to an IPv6 address and the device can establish a connection over IPv6 (that is, the destination is reachable over IPv6 and an IPv6 gateway is configured), then IPv6 is preferred.
• If the hostname only resolves to an IPv4 address, or if there is no viable IPv6 route (even if the hostname resolves to an IPv6 address), the device will use IPv4.

3. Protocol Preference

Some operating systems and applications have a preference for IPv6 when available. For example, the IPv6 address selection policy defined in RFC 6724 describes how devices should choose from multiple possible addresses to establish a connection, giving preference to IPv6 in many situations.

4. Independent Default Gateways

The default gateways for IPv4 and IPv6 are configured and operate independently. A device can have both an IPv4 gateway configured for IPv4 traffic and an IPv6 gateway for IPv6 traffic. The choice of using one or another gateway depends purely on whether the packet to be sent is IPv4 or IPv6.

5. Network Configuration and Support

The ability of a device to use IPv4 or IPv6 also depends on the configuration of the network to which it is connected. If the network only supports IPv4, then the device will only use IPv4, and vice versa. On networks that support both, the device can use both protocols, determined by the logic mentioned above.

In summary, a device configured with both IPv4 and IPv6 will use the appropriate protocol for each specific connection, based on availability, configuration, and protocol selection policies. There is no “default gateway” between IPv4 and IPv6; each protocol stack uses its own independently configured gateway.