There is actually some debate over the question of whether or not to spell Internet with a capital “I,” reflecting the fact that the English language is in a constant state of transition. If you are writing a specific or formal paper, you may want to consult the style guide preferred by your organization or educational institution, to ensure that you use the form of capitalization that will satisfy your readers.
The argument for the use of a capital I is that Internet is a proper noun, referring not to “an internet” but to a specific, worldwide network that uses the TCP/IP protocol. Under this argument, capitalization is extremely important, because it clarifies which network is under discussion. Many major publications around the world follow this stylistic edict, including technical journals and publications put out by organizations which work in the field.
When people who spell Internet with a capital “I” use the lower case, they are referring to “an internet,” a distinct interconnected network. For example, many offices and schools use an internet to connect their computers directly with one another. This can also be called an intranet. This system, whatever one calls it, is often also connected to the Internet — or the World Wide Web — just to add to the general confusion.
Other people believe that Internet should be spelled with a lowercase “I,” citing terms like “hoovering,” which were once proper nouns, but have now become so commonplace that they are not capitalized. Supporters of this argument point out that the word is not protected by trademarks and copyrights in the same what that many genericized phrases are, so it makes no sense to protect it with a capital “I” when this courtesy has not been extended to “spackle,” “nylon,” “zipper,” “velcro,” and numerous other products.
Spelling “Internet” with a lower case “I” is especially common in casual communications. However, many people believe that there should be a distinction between the Internet and an internet, as these two terms are very different. The best way to protect and clarify this distinction is to use a capital “I” when discussing the Internet, and supporters of this argument also capitalize terms like Web, Net, and WWW.