They don't need to be really long. Adding one more octet to v4 makes it 1 trillion addresses. Sure the header should support longer than that, but it doesn't mean everyone needs to use all of it.
1 trillion addresses isn't anywhere near enough[1]. Remember that we're not talking about L2 here. (...I don't think 1 trillion MACs would even be enough if we want to ensure they're unique, and it seems the IEEE agree with me since they expanded MACs to 64 bits, but that's a different conversation.)
And you already don't have to use all 128 bits in v6. You're forced to use about 48 of them, but the rest is your choice. You can just... not use some of them. It's fine.
How could you ever have really long addresses without having that problem?
They don't need to be really long. Adding one more octet to v4 makes it 1 trillion addresses. Sure the header should support longer than that, but it doesn't mean everyone needs to use all of it.
1 trillion addresses isn't anywhere near enough[1]. Remember that we're not talking about L2 here. (...I don't think 1 trillion MACs would even be enough if we want to ensure they're unique, and it seems the IEEE agree with me since they expanded MACs to 64 bits, but that's a different conversation.)
And you already don't have to use all 128 bits in v6. You're forced to use about 48 of them, but the rest is your choice. You can just... not use some of them. It's fine.
[1] https://www.rfc-editor.org/info/rfc1715/
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