A domain name usually consists of two or more parts (technically labels), separated by dots. The rightmost label conveys the TLD (top-level domain). Each next label to the left specifies a subdivision or subdomain of the domain above it. Note that “subdomain” expresses relative dependence and not the absolute one. In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
Label limits for IDN domain names should be applied to A-labels (translation to ASCII) instead of U-labels (non-ASCII Unicode representation). A name could be translated using IDN Conversion Tool for example.
The DNS consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains “beneath” it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root servers: the servers to query when looking up (resolving) a TLD (top-level domain name).