Pulling in missing data between spreadsheets

I receive a spreadsheet every week that has a list of IP addresses of servers that need updates. Not all servers need updates every week so this spreadsheet is dynamic. Some of the IP addresses on this weekly spreadsheet have the associated Hostname in an adjacent field, some do not. I have other spreadsheets that have the full listing of IP Address/hostname combinations.

What I want to do is take the spreadsheet that I receive every week and populate the missing hostnames from the other spreadsheet(s). So go down the list of IPs, and if the hostname field is empty, grab the name corresponding to that IP from one of the other spreadsheets.

I could combine the full listing of IP/hostname combinations into one spreadsheet easy enough if that simplifies things.