I Wanted an IPv4 Toast and Accidentally Built an IP Service

At 10:15 this morning I added a small feature to my personal website. Visitors whose providers still cannot deliver IPv6 would get a toast in the lower-right corner: Legacy IP detected You reached this website on a legacy version of the Internet Protocol (IPv4, 1981). It seems your provider does not support the current generation of Internet technology. Do ask them about it — IPv6 has only been a standard since 1998. Gently condescending, technically defensible, and dismissible. The dismissal lives in localStorage, so nobody gets lectured twice. I considered this an admirably restrained piece of web development. There was one problem. The website is static, so its JavaScript cannot ask the server which address connected to it. It had to ask somebody else. The first version used ipify. For most websites that would have been the end of the matter. For somebody who operates an autonomous system, announces his own address space, runs his own authoritative DNS, and has written an entire…