How to copy/paste a formula down an entire column in Excel 2011?
You need to use an absolute reference with $.

So that the cell references never changes when doing a copy and paste or fill change it from A1:B5 to $A$1:$B$5.

If you want you can even make a reference that is half absolute. The reference $D1 means that when you copy and paste the D will stay the same but the row number will change.