6to4 is an IPv4 to IPv6 migration scheme and one of the solutions for shifting network infrastructure from IPv4 to IPv6. The underlying concept of 6to4 is very similar to tunneling, but in the 6to4 construct, a node's given IPv4 address is appended to an IPv6 prefix, and all subsequent traffic is then transmitted to other 6to4 enabled nodes. This is most often accomplished via designated relay routers.
Techopedia explains 6to4
6to4 was devised as a way to transition from IPv4 to IPv6, which is a 128-bit addressing scheme. The first 64 bits are considered the address prefix, while the last 64 bits are designated for specific network hosts. Within the 64-bit prefix, a 6to4 enabled host's IPv4 address is appended to the first 16 bits in hexadecimal format. This is how the IPv4 address is transmitted to other nodes.
The Internet Assigned Numbers Authority (IANA) has designated the 2002::/16 prefix as the official prefix for all 6to4 traffic.