The Internet

OSI (The Open Systems Interconnection Model)
    and
TCP/IP (Transmission Control Protocol /Internet Protocol)

OSI

TCP/IP

Comparing OSI and TCP/IP

Comparing OSI and TCP/IP The TCP/IP Protocol Stack

Ports (Layer 4) -. Internet services listen on a particular port. For example, the World Wide Web Service listens on port 80. Whenever you go to a website, your browser is actually connecting to the particular computer on its port 80. Other services use other ports.

Here is a java.net.URL :

jdbc:mariadb://134.124.30.128:3306/test

134.124.30.128 is the ip address (Layer 3 ) of hoare.cs.umsl.edu. You could just use the name, requiring an address lookup.

You are connecting on Port 3306, which happens to be a MariaDB database. You are using Java Database Connectivity. The database is test.

Service Port Transport Protocol Description
ftp 21 udp File Transfer
ssh 22 tcp Secure Shell
telnet 23 tcp Telnet
smtp 25 tcp Simple Mail Transfer
http 80 tcp The World Wide Web