If you are not using inbound agents, it's recommended that you disable this entirely, which is the default installation behavior. Jenkins uses a TCP port to communicate with agents connected inbound. If you're going to use inbound agents, you can allow the system to randomly select a port at launch (this avoids interfering with other programs, including other Jenkins instances). As it's hard for firewalls to secure a random port, you can instead specify a fixed port number and configure your firewall accordingly.