How to access the subdomain of an IP address in the browser?

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.

If the aim is to serve several application on server without an assigned domain. Best way is to use ports

2.1.33.111:8080

This isn't possible via firefox. However a work around is to edit your OS hosts file see: wikipedia.

Create or edit and add a line like:

192.168.1.100 subdomain.example.com

Remember to remove the line when you're done!