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When IPv4 addresses are used up, Comcast will need to find a strategy for allowing a customer's IPv4-only devices to use an IPv6 address to communicate over an IPv4-driven Internet.
"We cannot force our customers to replace every single device in there homes. This is a non-starter," Durand says. "Also, if you look at the content on the Internet, the majority is reachable with IPv4. That may change in the future, but this is going to take many, many years."
The question is how Comcast can give its customers access to IPv4 content when there are no IPv4 addresses available. Unless customers upgrade their PCs to Vista, which is IPv6 enabled, they won't be able to reach IPv4 content without a new mechanism such as NATs for IPv6.
Comcast's idea is to allow many broadband customers to share one global IPv4 address instead of giving one global IPv4 address per customer.
"The exact ratio of IPv4 addresses to customers is something we are studying right now," Durand says. "We are working on some tests to see if it is 1 to 5, or 1 to 100 or 1 to 200."
This approach would be for new customers only; existing Comcast customers would keep the global IPv4 addresses they already have.
The trick with sharing public IPv4 addresses among many customers is doing it in the simplest way, Durand says.
One possibility involves two layers of NATs: one in customer gateways between private IPv4 addresses and shared public IPv4 addresses; and another inside carrier networks between shared IPv4 addresses and IPv6 addresses.
Durand says multiple layers of NATs would result in networks that are more complex and costly for carriers to operate.
"With two layers of NATs, there are two places where NATs can be tricky and create problems," Durand says. "Also your single view of the network is fragmented," which hinders debugging and repairs.
Instead, the Dual-Stack Lite approach would use one layer of NAT -- the carrier-grade NAT -- along with IPv4 to IPv6 tunneling from the customer's gateway to the carrier's NAT.
With Dual-Stack Lite, the carrier upgrades its networks to IPv6 but uses a combination of tunneling and NAT to allow customers with IPv4-only devices and IPv6 addresses to access IPv4 and IPv6 content.
New customers with IPv6 addresses would get special home gateways that do tunneling but not NAT. These gateways would take IPv4 packets and ship them over an IPv6 tunnel to the carrier-grade NAT, which handles translation in a way that's similar to today's IPv4 NATs.
"This greatly simplifies and reduces the cost of the home gateway," Durand says. He adds that the new home gateways would be dual stack, which means they support both IPv4 and IPv6.
The carrier-grade NAT would be a dual-stack router that terminates IPv4 to IPv6 tunnels and performs traditional IPv4 NAT. Durand says the carrier-grade NAT could be implemented on a PC running Linux,or it could be implemented in software only.
"We are talking to some open source developers about creating this," Durand says.