If you are just starting or building your website most people just go with the one .com however if begin to get lots of visitors and it becomes a very competitive market I would buy a range of domains such as .net or .org and so forth.

Theres not really much need to if you have a very unique niche, just make sure your aware of your traffic levels, competition status and domain rivals so you always win!

As for hypens being included in your domain name its a great method, the hypen is interpreted by Google Bot as a space so its great if it matches someones keyword in the search results.

Theres also services out there (Godaddy) for one that you can track multiple domains so that you find out if someone is interested in that domain, so say someone searches for yourdomain.net and you have yourdomain.com you get a log file for the number of searches and get a chance to register it before they do, its quite a useful techique.

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