A group of computers or other devices on the network managed as a single entity under common rules and procedures. In the context of the Internet, domains are defined by IP addresses. It is considered that all devices that share a common part of an IP address are within the same domain.
Domains come in three levels. Top-level domains exist for practically all recognized UN states and territories. They all consist of two letters, and the support for these domains in each country is handled by a special organization under ICANN's mandate. In addition to country code domains, there are a number of global top-level domains, such as .com, .net, .biz, and others. A second-level domain is a domain like www.address.ua, and a third-level domain is a domain like www.address.domain2.ua. In our example, .ua is the top-level domain.