Appropriate formula

[Adnan]

I have a spread sheet with 24000 records, and these records are divided by
category (each category has over 1000 records).
My question is, I need to assign a number per category, I tried Vlookup but
can’t seam to work it out.
I already have numbers per category in sheet2

i.e:
Sheet1 has the following
Category Assigned Nr.
Cat1 100
Cat1 100
Cat1 100
..
..
..
Cat2 300
Cat2 300
..
..
..

Sheet2 has this:
Category Assigned Nr.
Cat1 100
Cat2 300
Cat3 700



Thank you,
Adnan