A couple posts down, I was parsing the nslookup command to get hostnames. Even easier, use the host command. The hostname seems to be the fifth string after spaces, so using cut, it might look something like:
host <ip> | cut -d " " -f5
But there will be a period at the end, so just clean that up. Next is to get the IP and the hostname in some easy format, like colon or pipe delimited.