Change anything about IPv4, and you have to replace all the routers That's a broad oversimplification. Use the Cisco 6500 line as an example: it's a platform t

Change anything about IPv4, and you have to replace all the routers That's a broad oversimplification. Use the Cisco 6500 line as an example: it's a platform that's very long in the tooth, and through modularity is still functioning quite well, even for IPv6. Also: let's not pretend that similar issues weren't hit with v4, eg, the routing table explosion. You can change plenty of things without fork-lifting entire platforms, and serious long-term challenges exist even without considering v6.