missing data from table
 

Hi I have two tables both with approx. 5000 entries, Data Table A is correct
with 1-4999 but table two misses numbers out example 3500-3900 missing or
4000 -40006 missing how do I find out what numbers are missing by comparing
data from table A to the data in Table B? I have heard of lookup tables but
not sure how toi use these, any help much appreciated.