6over4

6over4: IPv6 over existing IPv4 networks

6over4 stands for "IPv6 over IPv4" and refers to a transition mechanism for migrating from IPv4 to IPv6. In this process, IPv6 packets are encapsulated within IPv4 packets and transported across an existing IPv4 infrastructure. This allows IPv6-enabled systems to communicate with one another, even when a native IPv6 network is not yet available between them.

The need for such methods arose due to the limited number of IPv4 addresses. IPv4 uses 32 bits, providing approximately 4.3 billion addresses. In contrast, IPv6 uses 128 bits, offering a significantly larger address space. Since the complete transition to IPv6 occurs gradually, IPv4 and IPv6 networks must operate in parallel for an extended period.

With 6over4, an IPv6 packet is embedded into an IPv4 packet. Transmission takes place directly via IPv4 protocol 41; no additional TCP or UDP ports are used. 6over4 can utilize IPv4 multicast to create a virtual, shared IPv6 network within an IPv4 network. In this context, IPv6 addresses are not simply created by prefixing IPv4 addresses with "fe80::"; rather, standard IPv6 mechanisms apply for specific addressing and neighbor discovery.

A prerequisite is that the participating systems must support 6over4, and the underlying IPv4 network must, in particular, support multicast traffic. Furthermore, routers and firewalls must permit IP protocol type 41. The additional encapsulation increases the size of the packet header, which can reduce the available payload. Improper configuration of the maximum packet size may lead to fragmentation or transmission issues. In practice, 6over4 is rarely used today. Native dual-stack networks, manually configured tunnels, or other transition mechanisms are far more common. Furthermore, from a security perspective, protocol type 41 must be monitored at firewalls, as 6over4 does not provide any built-in encryption.

For switches, the introduction of IPv6 generally does not represent a fundamental change, since they primarily forward traffic at the data link layer (Layer 2). It is crucial, however, that routers, firewalls, and management systems support IPv6. Consequently, while 6over4 served as a historical transition mechanism to enable IPv6 connections over existing IPv4 networks, it now plays only a minor role in modern networks.