a) stop growing, turn away customers (or, more likely, raise prices to drive out the lower paying customers to free up space for new customers)
b) buy up any organization they can find that has address space
c) go to the black market to obtain new address space at a market- defined price
d) accept new customers only if they come with their own address space
e) start selling (potentially multi-layer) NAT'd connectivity
Do you see any other options?
Most ISPs are large enough that they sell several different products to different customer bases. I expect that it will be more common to cannibalize less profitable product lines to get the IPv4 addresses needed for more profitable products. It's not the product price that is the factor because lower paying customers may be more profitable in aggregate. For instance if you have two broadband products, one which assigns a single IPv4 address using DHCP and one which provides a static /29, then the static product which has the higher price, may be the one which is cannibalized because it is not as profitable as leased-line sales using a /29. If you call this option f) then I would rank them in popularity: f) d) e) b) c) a) Choices f), d) and e) have a lot in common, i.e. they apply ingenuity to work around the problem rather than banging their heads against the brick wall of IP address shortage. Also, if you are enumerating choices in this way, it is devious to not include variations of deploying IPv6. Also, I expect that ISPs will come up with some variation of moving the NAT function into the ISP where they assign unregistered addresses to their customers, and then NAT this at a gateway on the ISP premises. With this variation, ISPs will reuse addresses that are registered to geographically distant organizations with whom their customers are unlikely to need to communicate. Since this provides a very large pool of IPv4 addresses that can be used more-or-less as RFC 1918-style addresses, there is enough flexibility to avoid problems. For instance, you could offer a range to a customer telling them who is registered to use that range, and if they veto it because they might need to communicate with that region, then you can pick another range. Perhaps we should not be talking about the brick wall of IPv4 exhaustion because it's more like a hall of mirrors. Some people will take their chances on navigating the hall and finding an exit, others will prefer to avoid the fun and just deploy IPv6. --Michael Dillon