Transition Mechanisms

What this chapter covers

This chapter explains how networks move from IPv4 to IPv6 without breaking what already works.

No network changes overnight. For many years IPv4 and IPv6 live side by side, so you need ways to run both and ways to carry IPv6 across parts of the network that do not support it yet.

The chapter covers dual stack, and the main tunnelling mechanisms: 6in4, 6to4, tunnel brokers, Teredo and ISATAP. It also separates tunnelling from translation.

How the topics fit together

Dual stack is the first choice, and tunnels fill the gaps where IPv6 is not available end to end.

The first page covers dual stack. Each device runs both protocols, and each stack has its own addresses, routes and firewall. The page shows how clients choose between an A record and an AAAA record, and why the IPv6 firewall must be configured on its own.

The second page covers tunnelling mechanisms. When the access network or a transit network supports only IPv4, a tunnel wraps IPv6 packets inside IPv4 packets. The page compares the manual 6in4 tunnel, the automatic 6to4 tunnel, tunnel brokers, Teredo and ISATAP.

The two pages are linked. A dual stack site that has no IPv6 from its ISP can use a tunnel to get connectivity, and the two methods are often used together.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

On the dual stack page, focus on what has to be duplicated and what cannot be shared between the two protocols. On the tunnelling page, build a comparison table from memory with the setup type, NAT behaviour and prefix for each mechanism.

The configuration of tunnels on RouterOS appears in the Interoperability chapter. Read this chapter first, because it explains why each tunnel exists.