Which ISP is that? It is very strange if they claim to be out of IPv4 addresses already, unless they of course did not bother to get enough IPv4 address space in the last few months. Depends more on what port/application you want to share. If it is just HTTP, then a HTTP Proxy will work perfectly fine, especially given that most websites use HTTP. Effectively your IPv4 address becomes what typically is designated as a load-balancer. Depends all on what you need that for. Depending on the version of Windows, the answer might be yes, this as they have Akamai'zed them some time ago, it depends though on Windows Update on the host itself it can use it.