Hi - I'd say there are no hard and fast rules here.

I usually advise clients with an existing business and recognised name only to buy the domains they need e.g. .co.uk and/or .com, .eu. I understand that the laws of 'passing off' apply online and if someone did register one of the other domain extensions and launch a site attempting to persuade people that they were that business any lawyer would soon put things straight. In my experience though it hardly ever happens that a domain name will be pinched in this way, though no doubt a few people out there have experience of it.

However if launching a new business online with new domains it may be a good idea to register a few more (.net, .biz, .info, .org if appropriate) but again to register all of the options does not seem necessary to me. It depends how comfortable you are with risk i.e. the very small risk that someone will come along and register the less popular domain extensions and set about trying to steal your traffic and/or impersonate your site versus the cost, which can become surprisingly significant if you do go to town registering names.

In addition you also need to consider the .mobi option as undoubtly this is a growing area of how people access information whist on the move.

It is neither practical nor fiscally sensible to attempt to own all domain extensions and domains that are variations on your theme unless you are a major organisation with deep pockets. If people want to steal your thunder they will always find a chink in your armour somehow if they are determined enough.

Nick brings up a very important point by talking about "brand protection" though. The only way you can truly protect your brand is through applying to the Patent Office to register your Trademark. This can be done by registering the words of your business name or the company logo which gives at least partial protection against someone stealing your identity.

Hope this helps.

Richard
Scribbles - Web Copywriting Services
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