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Comment by cesarb | Hacker News Reader

One thing I never understood in these proposals: how would two computers, one with only an extended address and the other with only an IPv4 address, talk to each other? Or two computers with extended addresses, but with a single router in the middle of their path which doesn't understand extended addresses?

All these proposals I've seen appear to assume that extended addresses start being distributed only after every or almost every host and router in the whole world had all of its software upgraded to understand extended addresses. But that's not realistic, since without being able to actually use it, there would be no incentive to modify every single piece of network-facing software and hardware to be able to use extended addresses. It's a Catch-22.

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There are two issues in migration: updating software and updating configuration. The former can be completely trivial if you are just pulling down updates from someone else. The latter requires effort on your part. If the IPv6 address space had been an extension, then you wouldn't have to do any configuration to support IPv6 clients while you maintain your existing 32-bit address, valid for both stacks.

At least that's my understanding of it.

For a new transport layer protocol like QUIC, this can work. However when you start talking about IP, you need to update/replace basically every device on the internet. If a device in the path doesn't know about the extended address space, that packet probably can't reach it's destination.

? You have to upgrade the software on all devices to move to a larger address space. That's understood. What we are discussing here is the added complexity of the configuration. Updating the software is easy by comparison.

> One thing I never understood in these proposals: how would two computers, one with only an extended address and the other with only an IPv4 address, talk to each other?

Via a NAT router that talks v4 on one side and v6 on the other.

A NAT router is not enough. Suppose the v4 side wants to initiate the communication; to which address would it send the initial packet? Remember, the "v4 side" has no concept of extended addresses at all, for it every address must be 32 bits and nothing more. And it also doesn't solve the "v4 router in the middle of the path" problem.

> A NAT router is not enough. Suppose the v4 side wants to initiate the communication.

The router would also need to act as a DNS server and translate v6 responses into dynamically assigned v4 addresses. Routers routinely do this sort of thing today for captive portals.

v4 systems can only talk to other v4 systems.

If there's a system that has only v4, it can tunnel v6 packets out to a tunnel gateway that will unwrap and forward them.

There are other protocol-specific tricks, like v4 DNS records that resolve to a HTTP reverse proxy, which forwards based on hostname and path to the real v6 server, while the v6 DNS records point directly to the v6 server.

> v4 systems can only talk to other v4 systems.

That depends on what you mean. A v4 packet can only be delivered unaltered to a v4 stack. But one can build a proxy server that tunnels TCP and UDP from a v4 network to a v6 network, so data can be passed from a v4 system to a v6 system without any software changes at either end.

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