Amazon has purchased several single-letter .CO domains.

The .CO domain extension is attracting interest from many business owners. Registration activity in this domain extension is very high, and many business owners see it as an alternative to the popular .COM domain extension.

Twitter recently acquired its .CO domain name, and yesterday, Amazon announced its interest in .CO domains, purchasing a.co, z.co, k.co, and cloud.co.

Twitter once received the t.co domain for free, and Amazon paid for these three .CO domains, though the value of the deal is unknown. Overstock once paid $350,000 for the O.Co domain. Godaddy also has its own short .CO domain, X.Co, which the company uses as a link shortening service.

The number of registrations in the .CO domain zone has now reached one million.

Amazon hasn't yet said why it needed such single-letter domains, although analysts note that a.co was likely purchased for Amazon, z.co for Zappos, and k.co for Kindle.

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The cloud.co domain may indicate the company's interest in launching a cloud music service.