I’ll let my profile on my family’s website introduce this one [warning: very outdated information can be found on this page - I'm talking about 6 years out here!].
John Christopher Nigel Gutteridge was born September 26 1988 5:30am. [...]. After a long debate by the family he was named John but informally called Jack.
My actual name is John. If I’m being formal, I’ll use John. So at work, my colleagues call me John for example. If I’m talking to my bank, registering something (you get the idea 
).
So let me set this straight, I’m called Jack but my actual name is John, right? My parents called me Jack, my family called me Jack, my friend called me Jack. Simple, right?
This used to prove very confusing to pupils in the same class as me back in the day. In Primary School it was okay, because I only had to explain the situation to one teacher a year. However, by Secondary School and into College I was having to explain myself to a tutor about once a week.
My fellow learners would be amazed and bewildered by this two-named freak
But it’s completely justified! All these morons thought I had some weird naming complex. Even the Wikipedia article on “Jack” agrees with me:
In English it is the diminutive form of the name John [...]
It was used back in the day (I’m talking around the Great War here) by a father called John calling his son John, but to avoid confusion would use the nickname Jack. Yeah?
So there! Get over it
Image source: BBC News Magazine – 7 questions on baby names
