Shodan Monitor will do it and if you're only keeping track of <= 16 IPs then you would just need the membership which is a one-time payment of $49 (i.e. no subscription cost) for a lifetime account upgrade (https://www.shodan.io/store/member). You just provide an IP/ network/ domain and we'll notify you if anything changes or becomes vulnerable. It's basically Google Alerts but for network ports:
We don't share revenue information but we've seen steady growth for the past 10+ years. All I can say is that Shodan is a profitable, bootstrapped business with >3.5 million users, 80%+ of Fortune 100 and thousands of universities (we let them setup monitoring for free for up to 120k IPs).
I'm using this and really love it, but it's annoying if you want to use a domain name instead of IPs, because if the DNS records change the old IPs stay in with the new ones.
That shouldn't be happening, can you confirm? It should flush out the old ones based on the current DNS information. The whole point is to automatically update the alerts based on the latest DNS information so you don't need to manage that yourself.
Sorry, that obviously shouldn't happen (both domain not updating and support falling through). We use Shodan Monitor for our own infrastructure and haven't seen that issue (or heard about it from our other customers) but I'll keep an eye out.
https://monitor.shodan.io
Disclaimer: I'm the founder of Shodan.