Why is HSTS (HTTP Strict Transport Security) not available on IP address URLs?

Question

I have implemented HSTS in my webserver configuration, and this is working as expected for domain URLs. However, when I test for HSTS availability using an IP address URL such as https://203.0.113.5/, HSTS is not available. Why is this? 

Answer

Per RFC 6797, HSTS hosts are identified by domain names. Identification by IP addresses is not available.

Further reading in this regard can be found below:

RFC 6797 - HTTP Strict Transport Security (HSTS)