The FMC and managed devices communicate using a two-way, SSL-encrypted communication channel, which by default is on port 8305.Cisco strongly recommends that you keep the default settings for the remote management port, but if themanagement port conflicts with other communications on your network, you can choose a different port. If you change the management port, you must change it for all devices in your deployment that need to communicate with each other.
Reference:
https://www.cisco.com/c/en/us/td/docs/security/firepower/misc/fmc-ftd-mgmt-nw/fmc-ftd-mgmtnw.html