Every blog needs a unique URL address, which visitors use to reach that blog. This address consists of a series of domain names, including a TLD (top level domain), such as .com and .org, and subdomains that come before it.
Domain names provided by Blogger and WordPress are free; however, if bloggers don’t like to include blogspot.com and wordpress.com in their blog’s domain name, they should buy a domain name, which usually costs $10 a year (with the exception of premium names containing very popular keywords, which may cost over $1000 a year).
Whether you use free domain name provided by Blogger and WordPress or paid domain name that you buy, it is important to make sure that the name that you choose must be SEO friendly. It must perfectly represent the topic of your blog, it must be rich in keywords that you target, and it must be easy to remember and to write.
The use of hyphens or underscores in a domain name is still subject to debates; however, although they may represent space, which prevents ambiguity of the meaning of the domain name but is not allowed in a URL, most bloggers discourage their usage because they make domain name more difficult to write and because domain names with no hyphen or underscore often ranks better than their hyphened and underscored counterparts.