systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units
Basic Port ListingHopefully someone finds this useful or at least interesting.
Name Port Protocol Description
Small services <20 tcp/udp small services
FTP 21 tcp file transfer
SSH 22 tcp login service
TELNET 23 tcp login service
SMTP 25 tcp mail
TIME 37 tcp/udp time synchronization
WINS 42 tcp/udp WINS replication
DNS 53 udp naming service........