I don't remember any variable-length encodings being proposed, but it was a long time ago. I don't think the 128-bit length was ever an issue, that is just a f

I don't remember any variable-length encodings being proposed, but it was a long time ago. I don't think the 128-bit length was ever an issue, that is just a field length, one parameter amongst many. It's all the other details that make a protocol (and it's surrounding infrastructure) more or less difficult to implement. One of the contending proposals literally modified an existing TCP/IP stack with a constant in all the relevant places, and was then compiled twice. It had #ifdef IPV6 #define ADDR_LENGTH 4 #else #define ADDR_LENGTH 16 #endif and then you could compile the same source twice, once for each protocol, just passing a -DIP6 on the command line for the IPV6 version.