Let's also assume that domain.com is hosted externally on Internet accessible DNS servers that are different from your internal DNS servers. For example, domain.com is hosted by GoDaddy or your ISP. So, on GoDaddy, you would have webapp.domain.com resolve to an external IP address such as 184.108.40.206.
Now, you can't create domain.com on your internal DNS servers. If you do, you'll need to manually synchronize all external DNS names to your internal DNS server. This can quite the pain to keep track of. We have several clients where the web hosting company is in control of the externally hosted DNS and change the Web site address without notice based on their infrastructure changing.
To resolve only webapp.domain.com differently, you create the webapp.domain.com as a forward lookup zone on your internal DNS. Then create a host record for the domain in that zone that resolves to the internal IP address, such as 192.168.1.100. When you are creating the record, it should look something like the graphic below.
While I'd like to say I'm the first person to think of this, I'm sure many have in the past. However, I learned this from the installation of SBS 2011 which does this by default for the remote.domain.com record that is used for accessing Exchange Web services and SharePoint.