* Blake Hudson
One thing not mentioned so far in this discussion is using PPPoE or some other tunnel/VPN technology for efficient IP utilization. The result could be zero wasted IP addresses without the need to resort to non-routable IP addresses in a customer's path (as the pdf suggested) and without some of the quirkyness or vendor lock-in of using ip unnumbered.
PPPoE (and other VPNs) have many of the same downsides as mentioned above though, they require routing cost and increase the complexity of the network. The question becomes which deployment has more cost: the simple, yet wasteful, design or the efficient, but complex, design.
<shameless plug alert> Or, simply just use IPv6, and use a stateless translation service located in the core network to provide IPv4 connectivity to the public Internet services. This allows for 100% efficient utilisation of whatever IPv4 addresses you have left - nothing needs to go to waste due to router interfaces, subnet power of 2 overhead, internal servers/services that have no Internet-available services, etc...all without requiring you to do anything special on the server/application stacks to support it (like set up tunnel endpoints), add dual-stack complexity into your network, or introduce any form of stateful translation or VPN service into your network. Here's some more resources: http://fud.no/talks/20130321-V6_World_Congress-The_Case_for_IPv6_Only_Data_C... http://tools.ietf.org/html/draft-anderson-siit-dc-00 In case you're interested in more, Ivan Pepelnjak and I will host a (free) webinar about the approach next week. Feel free to join! http://www.ipspace.net/IPv6-Only_Data_Centers BTW: I hear Cisco has implemented support for this approach in their latest AS1K code, although I haven't confirmed this myself yet. Tore