A subdomain is part of a domain in the domain name system. It is prepended to the domain as a prefix and separated by a period. Subdomains are aimed at guiding the user through the website. A subdomain is not registered but is established by the server. The use of a subdomain can structure it more clearly.
For example, subdomains are used when different products are distinguished from each other. For example, Google offers several services, such as Gmail or Google Docs. These are attached by subdomains. They also play an important role when the website contains pages that differ greatly in content. This is the case, for example, when a website uses different languages. It should also be noted that when using a subdomain, the www. is omitted.
English version: http://en.domain.com
Spanish version: http://es.domain.com
The relevance of subdomains increased when Google allowed only two results per host in the search engine rankings. This gave websites with several subdomains an advantage. It was also possible to take several places in the rankings of a search which caused more traffic, especially in the first ten places.
To date, subdomains play a significant role in ranking, even though individual subdomains are now grouped together by search engines.
Subdomains offer the possibility to logically organize the websites of larger organizations or companies. A large hotel chain, for example, could assign a subdomain to each hotel that belongs to the chain, thus logically structuring the various information about the hotels, such as the hotel and room descriptions, photographs, price lists etc., so that the user can quickly find his way around. This could look like this:
Starhotel.skyhotels.com Moonhotel.skyhotels.com Sunhotel.skyhotels.com
Since subdomains can be utilized to assist in navigation through a website, they are often compared to subdirectories. Subdomains which are not linked to the main domain and subdirectories are important for search engine optimization. Although they are not direct ranking factors, they play an important role. The decision whether which of the two is to be used, is up to the website operator. The differences in the configuration and the choice of the content management system should be considered.
Unlike subdomains, subdirectories do not require a domain name system. Moreover, it is easier to change the domain of a blog with a subdirectory. A domain change is possible and requires only the procurement of a new domain and subdomain.