Domains

What are Domains?

Domain names are the unique, human-readable Internet addresses of websites. They are made up of three parts: a top-level domain (sometimes called an extension or domain suffix), a domain name (or IP address), and an optional subdomain.

The combination of only the domain name and top-level domain is known as a "root domain." The "http://" is part of a page's URL but not its domain name and is known as the "protocol."
Let's look at each of these elements more closely:
Top-level domain
Top-level domain (TLD) is the formal term for the suffix that appears at the end of a domain name. Some example of top-level domains include:
-    .com
-    .net
-    .org
-    .edu

While we're probably all familiar with the TLDs above, there are actually over 1,000 possible TLDs from which webmasters can choose. This includes things like .book, .clothing, .dog, and .lifeinsurance (to name a few), as well as TLDs associated with specific countries or territories like .uk (United Kingdom) or .dk (Denmark). These country- (and sometimes region-) specific TLDs are known as country code top-level domains, or ccTLDs.


Domain name

Domain names are the second level of a domain's hierarchy (after the top-level domain). Domain names on a specific TLD (called a root domain, discussed below) are purchased from registrars and represent the specific, unique location of a website. In the following examples, the domain names are bolded:
www.example.o... engines do use the keywords in domain names as a ranking factor (which explains the existence of domains like "where-to-buy-the-best-donuts-in-Seattle.com"), but tread lightly when thinking about optimizing your root domain for search engines: One of the specific functions of Google's algorithm is to combat these keyword-stuffed exact-match domains.

Root domain
While the term "root domain" was originally created in the context of DNS (domain-name servers), it typically refers to the combination of a unique domain name and a top-level domain (extensions) to form a complete "website address." Your website's root domain is the highest page in your site hierarchy (probably your homepage). Individual pages or subdomains can be built off the root domain, but each page URL must technically include the same root domain in order to be a part of your website.
Examples of root domains include:
-    moz.com
-    Ilovedogs.net
-    PawneeIN.gov
All the pages on a single website have the same root domain (discussed below), and no two different websites can have the same root domain.
Because root domains represent whole websites instead of specific web pages, counting linking root domains instead of linking URLs (pages) can be a more accurate way to judge the size of your site's inbound link profile (generally speaking, more linking root domains is better). Backlink tools like Link Explorer or Majestic can help uncover the total number and specific identity of root domains linking to your site.

Subdomain
Subdomains are the third level of a domain's hierarchy and are parts of a larger top-level domain. They are added in front of the root domain and separated from the domain name with a period.
For example, "blog.example.com" and "english.example.com" are both subdomains of the "example.com" root domain. Subdomains are free to create under any root domain that a webmaster controls.
The two most common subdomain choices are:
-    http://www.example.com ("www" is the subdomain)
-    http://example.com (has no subdomain)

SEO best practices for domains
To maximize search engine-referred traffic, it is important to keep each of the following elements in mind:

1. Make your domain name memorable
Strive for domain names that are short, easy to remember, easy to type, and easy to say.  This is valuable for word-of-mouth advertising because those visitors will need to visit your domain directly, but it also matters for processing fluency. An implicit cognitive bias, processing fluency is the concept that we remember and have more positive associations with things that we can easily say and easily think about, and that includes pronounceability in our own minds. So, stay away from domain names that include numbers or other non-standard characters, use unusual spelling, or are longer than about 15 characters or so.
Because of search engine's growing reliance on accessibility and usability as a ranking factor, the easier a domain (or URL) is to read for humans, the better it is for search engines.


2. Use broad keywords when sensible
If you can include a keyword that helps make it obvious what your business does while keeping your domain name catchy, unique, and brand-friendly, go for it. But, stay away from domain names that might be considered "keyword-rich" or "keyword-targeted" (such as best-pancake-pans-for-pancakes.com or senior-eldercare-retirement-home-finder.com) We mentioned this earlier, but it's worth mentioning again: While these types of domain names once carried weight as a ranking factor, their tendency to be associated with low-quality content means searchers (and search engines) may now view these keyword-dense domain names with a negative bias.
What's more, in recent years Google has made several changes that have de-prioritized sites with keyword-rich domains that aren't otherwise high-quality. Having a keyword in your domain can still be beneficial, but it can also lead to closer scrutiny and a possible negative ranking effect from search engines, so tread carefully.

3. Avoid hyphens if possible
If your domain name is two words (like www.examplesite.com), you may want to separate the words with a hyphen for readability: www.example-site.com. But, keep in mind that use of hyphens also strongly correlates with spammy behavior and decreases domain name readability and memorability. For that reason, generally, no more than one hyphen should be used (if any must be used at all). 

4. Avoid non-.com top-level domains (TLDs)
When a webmaster registers a domain name, they will often be given the option to buy additional TLDs. In order to maximize the direct traffic to a domain, bias towards purchasing the .com TLD. If the .com TLD for a domain name you're looking to purchase isn't available, Rand recommends leaning towards .net. , .co, or a known ccTLD as alternatives.
Additionally, it is not recommended that SEO-conscious webmasters purchase low-quality TLDs such as .biz, .info, .ws, .name, etc. as a means of increasing traffic. Because they're less commonly known, these TLDs receive substantially less traffic than the more widely known domains as well as may be more frequently associated with spammy behavior.

5. Favor subfolders/subdirectories over subdomains
Search engines keep different metrics for domains than they do for subdomains, so even though Google itself has stated that from a ranking perspective content in subdomains and subdirectories is treated roughly equally, it's still recommended that webmasters place link-worthy content like blogs in subfolders rather than subdomains (i.e. www.example.com/blog/ rather than blog.example.com).
The notable exceptions to this are language-specific websites. (i.e., en.example.com for the English version of the website).

6. Don't sweat over domain age
The notion that the age of a domain (i.e. how long it's been registered) is an important SEO ranking factor is a myth. When asked about domain registration length, Google's Matt Cutts said, "To the best of my knowledge, no search engine has ever confirmed that they use length of registration as a factor in scoring. If a company is asserting that as a fact, that would be troubling. The primary reason to renew a domain would be if it's your main domain, you plan to keep it for a while, or you'd prefer the convenience of renewing so that you don't need to stress about your domain expiring."
What does matter to Google (albeit to varying degrees) are things like how long it’s been since your site was first crawled or since the first inbound link was recorded. Even so, though, Matt Cutts assures that "the difference between a domain that's six months old and one year old is really not that big at all."

7. Moving domains
If a webmaster needs to move one domain to another, there are several critical factors to consider, including setting up the redirects on a page-to-page basis such that sub-folders and deep content pages are redirected to corresponding sub-folders and deep content pages on the new domain. SEOs should avoid redirecting all pages from one domain to the homepage of another domain.