To get to the IP address of an example website, you just visit
subdomain.example.com
However, if I try to visit
subdomain.2.1.33.111 (example ip)
Firefox returns an error.
Why?
An IP address with a subdomain is not a valid statement - an IP address is not a domain, thus it cannot have a subdomain. It is not possible without DNS.
All browsers will return an error for this. The reason is that subdomains are part of the DNS (Domain Name Service) system, where IP addresses are related to the underlying IP protocol.
The best way to think of this relationship is that domains (including subdomains) are human-readable labels which DNS then allows you to point to IP addresses. It would not be very catchy to have an IP address as your website on a TV ad, for example.
There is much more detail on DNS and IP addresses if you want to delve into more detail than this.