Referencing Multiple Workbooks

I have a 'dashboard' workbook that includes summary data from multiple other workbooks that my co-workers edit in a network drive. I have it set up to reference the cells in the other workbooks.

To reference the data in the dashboard I click in a cell, press '=' then go to the other workbook and click the cell I want to reference and press enter. For example, the formula looks like ='http://168.1.3.5//files/[workbook1.xls]Sheet1'!D$21

This works great, but it is way too time consuming b/c sometimes I may have hundreds of workbooks I need to reference.

The cell I need to reference is the same cell in each workbook, and each workbook has the exact same network directory and file name with the exception of a number in the file name. i.e. There is workbook1, workbook2, workbook3, etc.

I would like to find a way to simply reference the first workbook, then use the drag feature to reference the rest of the workbooks. Unfortunately, because it is a reference, and not a mathmatical forumla, I haven't figured out how to let excel know to keep the link the same and just change the number in the referenced filename

='http://168.1.3.5//files/[workbook1.xls]Sheet1'!D$21

I need to change the "1" in "workbook1" each time I drag down, but I can't seem to get this to work. (i.e. workbook1 - 1 needs to change in order 2, 3, 4 as I drag down). I've tried using concontanate to sting the link together but I cannot get that to work either. Also, my company doesn't allow me to download Template Wizard but I can use Macros.

Any ideas? Thanks for your help!