I disagree with many of the previous posters who say that
Jack is good as a nickname for
Jackson. I disagree, as
Jackson is a trendy "surname turned first name" that will sound a bit dated after the fad wears off. If this were my child, I would give them the legal name
John and the nickname
Jack, as it gives the child more options later in life, and I think that
John sounds more professional than
Jack, but
Jack as a legal name would probably be fine as well, since now
Jack has become a name in itself, in addition to being a nickname for
John.