Both are proxies of a sort. They're effectively websites with a wildcard DNS entry and the subdomain is then used to determine the remote site. In nyud.net's case, this is located in their CDN before the real site is contacted. In the case of sixxs.org, the site is contacted directly. The difference is in the DNS entries. .ipv4.sixxs.org returns an IPv4 address and .sixxs.org returns an IPv6 address. Once connected, if the Host: header ends in ipv4.sixxs.org, the proxy connects to the remote site over IPv6; if not, the proxy connects over IPv4. The end result is that you get a proxy that lets you access the IPv6 web without IPv6 (and the IPv4 web without IPv4).