ISATAP

ISATAP (intra-site automatic tunnel addressing protocol) is an IPv6 transition mechanism meant to transmit IPv6 packets between dual-stack nodes on top of an IPv4 network.

ISATAP typically builds its Potential Router List (PRL) by consulting the DNS; hence, in the OSI model it is a lower-layer protocol that relies on a higher layer.

[3] Due to a patent claim, early in-kernel implementations were withdrawn from both KAME (*BSD) and USAGI (Linux).

However, the IETF IPR disclosure search engine reports that the would-be infringing patent's holder requires no license from implementers.

For prior kernels, the open source project Miredo provided an incomplete userland ISATAP implementation, which was removed in version 1.1.6.