and any associated information of assignees, registrants, administrative information, such as creation and expiration dates. Two data models exist for storing resource information in a WHOIS database, the thick and the thin model. Thin and thick lookups WHOIS information can be stored and looked up according to either a thick or a thin data model: Thick A Thick WHOIS server stores the complete WHOIS information from all the registrars for the particular set of data (so that one WHOIS server can respond with WHOIS information on all .org domains, for example). Thin A Thin WHOIS server stores only the name of the WHOIS server of the registrar of a domain, which in turn has the full details on the data being looked up (such as the .com WHOIS servers, which refer the WHOIS query to the registrar where the domain was registered). The thick model usually ensures consistent data and slightly faster queries, since only one WHOIS server needs to be contacted. If a registrar goes out of business, a thick registry contains all important information (if the registrant entered correct data, and privacy features were not used to obscure the data) and registration information can be retained. But with a thin registry, the contact information might not be available, and it could be difficult for the rightful registrant to retain control of the domain. If a WHOIS client did not understand how to deal with this situation, it would display the full information from the registrar. Unfortunately, the WHOIS protocol has no standard for determining how to distinguish the thin model from the thick model. Specific details of which records are stored vary among domain name registries. Some top-level domains, including and , operate a thin WHOIS, requiring domain registrars to maintain their own customers' data. The other global top-level registries, including , operate a thick model. Each country-code top-level registry has its own national rules. Software The first applications written for the WHOIS information system were command-line interface tools for Unix and Unix-like operating systems (i.e. Solaris, Linux etc.). WHOIS client and server software is distributed as free open-source software and binary distributions are included with all Unix-like systems. Various commercial Unix implementations may use a proprietary implementation (for example, Solaris 7). A WHOIS command line client passes a phrase given as an argument directly to the WHOIS server. Various free open source examples can still be found