The endings of the domain name dictate which organisation controls them. In practice, this is the organisation that controls the name servers for that domain. Therefore ICANN has de facto control of the overall Domain Name System because it controls the root name servers.
Registries make the index available to the world via Whois systems and via their name servers, for the direction of internet traffic. Such systems have to be fully redundant because loss of name servers can affect all internet traffic sent to that domain.
The final section of the name '.com', '.biz', '.uk', '.de' is called the top-level domain (TLD). Some exhibit no affilation with a particular country (like .com) and are called generic Top Level Domains (gTLD). These are operated by registrars appointed by ICANN. However, in addition, every country in the world has a two letter code (a country code TLD or ccTLD).
These ccTLDs are operated by a range of organisations: some are not-for-profit commercial organisations, others are government departments. Some have signed a contract with ICANN, some have not.
These ccTLDs also vary in size: while .com has [at end-2003] about 21 million registrations spread over the various registrars, .de has some 6.5 million controlled by DENIC (the .de registry) alone and the .uk registry Nominet UK has about 4.5 million. Other ccTLDs may have as little as a few hundred.
The registry will hold the central register and operate the name servers for that domain. They will generally set policies for the names it controls: it may restrict certain names for political, religious, historical or local legal reasons.
Equally, ccTLD registries set the dispute policies for their names: those that have signed up with ICANN genrally have to use the UDRP, while Denic require people to use the normal German civil courts, and Nominet UK deal with Intellectual Property and other disputes through its specific dispute resolution service.
The ccTLD registries can also control whether matters of interest to their local communities are intruduced: for example, the Japanese and Polish registries have introduced internationalised domain names to allow use of local non-ASCII characters.
In the gTLD system there are not really any unified registries in the same way, although the Public Interest Registry which runs .org is close. In the gTLD system ICANN holds a basic register which records the name, other critical details, and which registrar (agent of the registry) runs that name. The registrar holds the other details like the registrant's telephone number.
Operation of registries
Registries are run in many different ways. Some are government departments (e.g. the registry for the Vatican www.nic.va). Some are co-operatives of internet service providers (such as DENIC www.nic.de) or not-for profit companies (such as Nominet UK www.nic.uk). Others are commercial organisations, such as the US regstry (www.nic.us). For certain repressive countries, control over the registry and ISPs can effectively control entirely what access their citizens have to the Internet.
Registries operate all sorts of systems in order to hand out names. Generally registries operate a 'first-come-first-served' system of allocation. Some registries sell the names directly and others rely on ISPs or registrars to sell them. All registries will have rules about which domain names can be registered. Some of these rules are technical, and so universal, but many are cultural, or depend on the nature of the registry. For example, registries differ hugely in their attitude to obscene or libellous domain names.
The level of charges depends on the nature of the Registry - commercial registries naturally tend to charge what the market will bear, whereas non-commercial registries tend to charge less.
Registries may also impose a system of second level domains on users. The argument for such domains is that it allows more space and certainty in the system. Thus e.g. governemental organisations cannot be impersonated and individuals can have a domain name separate to that given to companies. The argument against is that it leads to less memorable names and fragments the system.
The contrasting approach can be seen in three of Europe's biggest registries. For example, DENIC, the registry for Germany (.de) does not impose second level domains. AFNIC, the registry for France (.fr) has some second level domains, but not all registrants have to use them, and Nominet UK, the registry for the United Kingdom (.uk) requires all names to have a second level domain.