As the MLA Handbook (2.5.2) notes, “When giving a URL,” or web address, “copy it in full from your web browser.” Thus, a web address should generally be set roman and styled lowercase:

The search engine can be found at google.com.

Note, however, that a website’s address should not be confused with its title. In MLA style, you should use the title of a website as it appears on the site and italicize it as you would any independent work. Do not use the web address as the title unless the address and the title are identical. The following example shows the distinction:

We were looking at the differences between Google, located at google.com, and Yahoo!, located at yahoo.com.

But if you are quoting a source or editing an interview, and the speaker refers to websites by their addresses, you should leave the information as is:

The interviewee said that they “were looking at the differences between google.com and yahoo.com.”

Work Cited

MLA Handbook. 8th ed., Modern Language Association of America, 2016.