Preferences > Ports
Server programs, when addressed over the network, must be assigned to a specific network port. This allows multiple server programs to run on a single server machine. Each service has a default port: The Apache web server typically uses port 80, and the MySQL database server uses port 3306.
These ports are configurable. The default configuration for MAMP uses ports 8888 and 8889, as well as 7888. This allows the MAMP servers to run alongside other servers installed on your Mac. If ports 7888, 8888, or 8889 are being used by another application, please change the values accordingly.
The button Set Web & MySQL ports to 80 & 3306 will set the ports to the value commonly used on the Internet. The button Set MAMP ports to default will reset the ports for Apache, Nginx and MySQL to 8888, 7888 and 8889.
If you want MAMP to be available over the Internet, make sure that the configured ports are open in your firewall.