sounds like you're talking about CORS. you can list domains that ajax is allowed to fetch using the "Access-Control-Allow-Origin" header.
there's also the document.domain property which tells you your current domain. If you had a list of domains, you would use document.domain to spot that domain in a lineup.
(2013/5) HTML5:90.2% MOB:15.2% IE7:0.5% IE8:8.4% IE9:8.5% IE10:8.5%