> […] but can easily be traversed if you understand the extension. You need new code to "understand the extension". Any old network stack will not, and will

> […] but can easily be traversed if you understand the extension. You need new code to "understand the extension". Any old network stack will not, and will thus not be able to send packets to it… just like old code does not understand IPv6 addresses. > What am I misunderstanding / oversimplifying? IPv4 is 32 bits, and all the data structures are 32 bits. We are running out of 32 bit addresses. If you want more address space you have to have more than 32 bits, and it is impossible to squeeze >32 bits in a 32 bit data structures, last time I checked. So you need to ship code on every single Internet device to update it to handle >32 bits. We've just spent the last few years shipping new code for larger address data structures, i.e., the 128 bits of IPv6. Look how long that's taken. And you you want to do that again?