The public suffix list contains points in the DNS where (roughly speaking) names below that point are under different management from each other and from that name. It's here: http://publicsuffix.org/ The idea is that abc.foo.com and xyz.foo.com have the same management, but abc.co.uk and xyz.co.uk do not. You don't have to tell me that it's a gross crock, but it seems to be a useful one. What do people use it for? Here's what I know of: * Web browsers use it to manage cookies to keep a site from putting cookies that will affect other sites, e.g. abc.foo.co.uk can set a cookie for foo.co.uk but not for co.uk. * DMARC (www.dmarc.org) uses it to find a policy record in the DNS that describes a subtree, e.g., if you get mail that purports to be from eBay@reply1.ebay.com it checks the policy at ebay.com. What other current applications are there? R's, John PS: Really, you don't have to tell me what a crock it is.