Password Protect Blog depending on IP

Hi,

I am new to wordpress and also learning PHP. I work as a Graduate Web Developer for a company that is interested in doing a Blog that would be used for the business to collaborate some research etc. I am doing ok with the theming etc as I am creating a child theme of the Twenty Eleven theme. However my problem lies with the fact that they want everyone in the office (According to my manager it stays within a certain IP Range) to be able to access the blog but if you are outside of the office (outside of the IP Range) then you are forced to be logged in to access the site. I know there is a plugin to force you to login but how do I manage to restrict the forced login to the IP range that my manager has?
Could someone please help me. Thank you so much