Title: Domain Name System
Content:  The definitive descriptions of the rules for forming domain names appear in RFC 1035, RFC 1123, RFC 2181, and RFC 5892. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example.com. The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com. The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example, the label example specifies a subdomain of the com domain, and www is a subdomain of example.com. This tree of subdivisions may have up to 127 levels. A label may contain zero to 63 
Title: Domain name
Content:  A domain name is an identification string that defines a realm of administrative autonomy, authority or control within the Internet. Domain names are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name identifies a network domain, or it represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a server computer hosting a website, or the web site itself or any other service communicated via the Internet. In 2017, 330.6 million domain names had been registered. Domain names are formed by the rules and procedures of 
Title: Domain name
Content:  be resolvable either in the new domain, or exist elsewhere. Based on traditional requirements (RFC 1034), typically a minimum of two servers is required. In the process of registering a domain name and maintaining authority over the new name space created, registrars use several key pieces of information connected with a domain: A domain name consists of one or more labels, each of which is formed from the set of ASCII letters, digits, and hyphens (a-z, A-Z, 0–9, -), but not starting or ending with a hyphen. The labels are case-insensitive; for example, 'label' is equivalent to 'Label' or 'LABEL'. In the textual representation of a domain name, the labels are separated by a full stop (period).
Title: Domain name
Content:  Domain Name System (DNS). Any name registered in the DNS is a domain name. Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other 
Title: Domain name
Content:  Domain names serve to identify Internet resources, such as computers, networks, and services, with a text-based label that is easier to memorize than the numerical addresses used in the Internet protocols. A domain name may represent entire collections of such resources or individual instances. Individual Internet host computers use domain names as host identifiers, also called hostnames. The term hostname is also used for the leaf labels in the domain name system, usually without further subordinate domain name space. Hostnames appear as a component in Uniform Resource Locators (URLs) for Internet resources such as websites (e.g., en.wikipedia.org). Domain names are also used as simple identification labels to indicate ownership or control of a resource. Such examples are the realm identifiers 
Title: Domain name
Content:  Below the top-level domains in the domain name hierarchy are the second-level domain (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain example.co.uk, co is the second-level domain. Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An example of an operational domain name with four levels of domain labels is sos.state.oh.us. Each label is separated by a full stop (dot). 'sos' is said to be a sub-domain of 'state.oh.us', and 
Title: Domain name
Content:  limit is 127 levels: 127 characters plus 126 dots have a total length of 253. In practice, some domain registries may have shorter limits. ; A hostname is a domain name that has at least one associated IP address. For example, the domain names www.example.com and example.com are also hostnames, whereas the com domain is not. However, other top-level domains, particularly country code top-level domains, may indeed have an IP address, and if so, they are also hostnames. ; Hostnames impose restrictions on the characters allowed in the corresponding domain name. A valid hostname is also a valid domain name, but a valid domain name may not necessarily be valid as a hostname. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example.com.
Title: Domain name
Content:  accessible Internet resources or run web sites. The registration of these domain names is usually administered by domain name registrars who sell their services to the public. A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Traditionally a FQDN ends in a dot (.) to denote the top of the DNS tree. Labels in the Domain Name System are case-insensitive, and may therefore be written in any desired capitalization method, but most commonly domain names are written in lowercase in technical contexts.
Title: Domain Name System
Content:  The domain name space consists of a tree data structure. Each node or leaf in the tree has a label and zero or more resource records (RR), which hold information associated with the domain name. The domain name itself consists of the label, concatenated with the name of its parent node on the right, separated by a dot. The tree sub-divides into zones beginning at the root zone. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative choices of the zone manager. DNS can also be partitioned according to class where the separate classes can be thought of as an array of parallel namespace trees.  Administrative responsibility for any zone may be divided by creating additional zones. Authority over the new zone is said to be delegated to a designated name server. The parent zone ceases to be authoritative for the new zone.
Title: Subdomain
Content:  allocate a two-level domain name. Hosting services typically provide DNS Servers to resolve subdomains within that master domain. A fully qualified domain name consists of multiple parts. For example, take the English Wikipedia domain. The  is a subdomain of. Although  is usually considered to be the domain name,   is actually a sub-domain of the   TLD (top level domain). Any fully qualified domain name can be a host or a subdomain. A domain name that does not include any subdomains is known as an apex domain, root domain, or bare domain. For example,  is the apex domain of Wikipedia, which redirects to the subdomain.

Question: what are the parts of a domain name called?
Answer: subdomain.
top-level domain.
hostname.