Range Continue Until It Recognizes The Last Cell

I have this macro that does an autofill in Column A depending on what is in Column B, however the rows are going to increment frequently. Column B has repeating numbers but they are not all even amounts. So column B can 1,1,1,1,2,2,2,2,2,3,3,4,4,4,4,4,4,4,4,5,5, and so on all the way down. The code will identify when the value changes to something new and in Column A will start numbering from 1 to whatever until it recognizes that a value changes in Column B.

Columns("A:A").Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Columns("C:C").Select
Selection.Cut
Columns("A:A").Select
ActiveSheet.Paste
Columns("C:C").Select
Selection.Delete Shift:=xlToLeft
Columns("A:A").Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Range("A1").Select..............................

The problem that I have is in bold with the range. I need it to continue until it recognizes the last cell in the column that has a value and stop.

Right now it will work but only until row A6391 for my first sheet however these rows will increase over time. I'm using Office 2007.