IPv6 was designed to be compatible to IPv4.
Sort of. But not really.
The addressing scheme is an extension to IPv4. Instead of an IP address that has xxx.xxx.xxx.xxx, you will have XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX
There are a special set of addresses that will understand XXXX:XXXX:XXXX:XXXX:xxx.xxx.xxx.xxx but not all devices will necessarily support it as the RFC does not require it.
Additionally, while the addressing scheme MAY be somewhat compatible, there are a host of issues regarding compatibility that were never addressed by the design committee.
For instance:
1. A router with the drivers in Read Only Memory. While IPv6 support may be available, in order to implement it on the device the device will have to be taken apart, the ROM shipped back to the manufacturer and re-burned.
2. A router that was built without IPv6 support and the manufacturer either will not provide upgrade software or is out of business.
3. A router that doesn't have enough memory to process the routing tables for the larger address space.