The hierarchy of domains descends from the right to the left label in the name; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a node example.com as a subdomain of the com domain, and www is a label to create www.example.com, a subdomain of example.com. Each label may contain from 1 to 63 octets. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a dot. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.[6] Thus, when using a single character per label, the 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 handful of domains will have restrictions on them, which means you can only purchase them if you meet or exceed certain criteria or have authorization (some examples are .gov, .edu and .mil). But most extensions are available to everyone. In fact, most country code Top Level Domains (ccTLDs) are available for anyone to purchase, even if you don't reside in the country code in question.
Domain names are formed by the rules and procedures of the 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 publicly accessible Internet resources or run web sites.
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).

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 web sites (e.g., en.wikipedia.org).
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 'state' a sub-domain of 'oh.us', etc. In general, subdomains are domains subordinate to their parent domain. An example of very deep levels of subdomain ordering are the IPv6 reverse resolution DNS zones, e.g., 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which is the reverse DNS resolution domain name for the IP address of a loopback interface, or the localhost name.
When you build a website, you want visitors to come and see what you've done. To get them there, you need a unique domain name that connects to your sites servers. Domain name registration is required to ensure that no one else in the world can claim ownership of your web site's address and to make finding your website simple. Find your one of a kind domain name.
Second-level (or lower-level, depending on the established parent hierarchy) domain names are often created based on the name of a company (e.g., bbc.co.uk), product or service (e.g. hotmail.com). Below these levels, the next domain name component has been used to designate a particular host server. Therefore, ftp.example.com might be an FTP server, www.example.com would be a World Wide Web server, and mail.example.com could be an email server, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. load balancing) or even identical addresses (cf. anycast) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in Web hosting service centers, where service providers host the websites of many organizations on just a few servers.
Think of the name you want to register. The answer is typically your company or website name. It is best to keep your domain name short and easy to understand. Say it out loud, and make sure it sounds great. Next, search to see if it is available. If the name you desire is taken with the .com top-level domain, there are hundreds of others available. Finally, add the top choices to your cart and complete the domain registration.
“HubSpot’s blog is a well-trafficked resource because we put a lot of thought into posting  valuable content for our customers, partners and even employees or future employees, so culturecode.com was a natural place to unveil our culture code. However, we also wanted the ability to easily share the code, and giving someone a lengthy URL or asking them to do a search on our site isn’t very convenient.  On the other hand, a custom domain is much easier to remember.”
Think of the name you want to register. The answer is typically your company or website name. It is best to keep your domain name short and easy to understand. Say it out loud, and make sure it sounds great. Next, search to see if it is available. If the name you desire is taken with the .com top-level domain, there are hundreds of others available. Finally, add the top choices to your cart and complete the domain registration.
The Internet Corporation for Assigned Names and Numbers (ICANN) is the organization that governs the rules and regulations for domain name registrations. ICANN requires, for various reasons including to determine ownership of a domain should a dispute transpire, that a publicly accessible database be maintained that contains the contact information of all domain registrants. In layman's terms this means your domain name will be searchable by anyone and those search results will include your full name, physical address and other contact information. In order to protect your privacy in this regard, Domain.com offers WHOIS Domain Privacy which then masks your information using our own and implements a procedure for you to control who is able to then gain access to your contact information via a WHOIS search. Whenever you buy a domain name, no matter what domain name registration service you use, you are subject to the same ICANN rules, for this reason it is important to use a reputable service who cares about your privacy. Domain.com always recommends enabling WHOIS Domain Privacy.
Think of the name you want to register. The answer is typically your company or website name. It is best to keep your domain name short and easy to understand. Say it out loud, and make sure it sounds great. Next, search to see if it is available. If the name you desire is taken with the .com top-level domain, there are hundreds of others available. Finally, add the top choices to your cart and complete the domain registration.
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. 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.[2]

The hierarchy of domains descends from the right to the left label in the name; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a node example.com as a subdomain of the com domain, and www is a label to create www.example.com, a subdomain of example.com. Each label may contain from 1 to 63 octets. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a dot. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.[6] Thus, when using a single character per label, the limit is 127 levels: 127 characters plus 126 dots have a total length of 253. In practice, some domain registries may have shorter limits.


Name servers. Most registrars provide two or more name servers as part of the registration service. However, a registrant may specify its own authoritative name servers to host a domain's resource records. The registrar's policies govern the number of servers and the type of server information required. Some providers require a hostname and the corresponding IP address or just the hostname, which must be resolvable either in the new domain, or exist elsewhere. Based on traditional requirements (RFC 1034), typically a minimum of two servers is required.
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