Assigning Values from Fixed Data Set to Variable Data Set

So if I have fixed data like this: (2 columns always, up to 50K rows)

00001 R
00002 S
00003 U

Is there a formula, vba, pivot, ANYthing that I can use so that when I get a new list of, in this case, numbers, I can automatically assign it the correct letter? I basically have a list of every ZIP code in the US, and it's corresponding classification. When I get a list of ZIPs with no classification, I have to assign them from the existing list. I can do this with sorting and a specific formula, but I wanted to know if there is something easier as this method can get a bit tedious. Something where I can just paste in the new data and a column will populate with the correct designation....

Thanks in advance!