It was first introduced by the National Insurance Act 1911, and expanded by the government of Clement Attlee in 1946.
I Googled `National Insurance Number`. Whilst a lot of the results are for obtaining a current number there is a useful entry on Wikipedia. It explains how numbers are allocated and made up, which might be helpful?http://en.wikipedia.org/wiki/National_Insurance_number
What is it you`d like to find out about the person who`s number you have? I think tracing their family history in the usual way would be easier.
Useful thread here http://www.british-genealogy.com/forums/archive/index.php/t-...
Hope this helps