# Coexistence with Legacy IPv4

The notion of a utopian IPv6-only world is a noble goal, however, as with any tectonic change, it happens slowly, and differing elements exist simultaneously.  As such, expectations should be set that in many cases coexistence with legacy IPv4 is the norm, and while it should be considered a transitional state, it may exist for extended or indefinite periods of time. Reasoning for coexistence will vary and is typically only locally relevant to a given environment. It may be due to the requirement for legacy hardware with no IPv6 support that requires capitol expenditure beyond the budget of an organization, such as a specialized piece of operational technology, or it may be due to lagging compliance regulations that have not tracked current technology standards. Regardless, the reasonings are less important than the details necessary to support a dual-stacked environment.

As networks migrate away from IPv6 and into an IPv6-only environment, they will undoubetedly discover unexpected hurdles consisting of half-completed protocol stacks, lack of capabilities, and unexposed configuration knobs. These will almost certainly be discovered in the periphery of the network. Elements such as power controllers, optical multiplexing platforms, mechanical control systems, and other speciality hardware tend to posess a very long mean time to replacement, and a slow to modernize firmware offering. Operational technologies and SCADA systems are also very slow to update and may also live in the network for many years, if not decades.   

- IPv4 only enclaves
- IPv4-IPv6 dual stack
    - Supporting ships in the night protocols
        - Consistent policy
        - Monitoring and measurement
        - Multi-topology within the internet
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## [Dual stacks](Dual%20stacks.md)
## [Tunnels](Tunnels.md)
## [Translation](Translation.md)
## [Obsolete techniques](Obsolete%20techniques.md)
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