Here I have a table of 13075 ports that hackers have been trying to access on one of my servers in one week. Many are well known ports. Most are rather random... This clearly shows you why you want to have a firewall properly setup on your computer. If you run any local services (all computers do) then hackers can connect to them unless you block them with a strong firewall setup.

For sure, if you see a port or service name that you recognize (that runs on your computer), then know that anyone on the Internet will be able to connect to it if you do not have a firewall in place. If you're an individual, it is very likely that you'll get a preset firewall in your router and possibly from your ISP too.

In order to see whether a port is open on a Linux system, you can use the following commands:

TCP Ports:

netstat -a64n | grep LISTEN

UDP Ports

netstat -a64nu

If the Foreign Address says "0.0.0.0" or has your current Internet IP address, the people from anywhere can connect to that service unless it is blocked by the firewall. If the address is 127.0.0.1, then only other software running on that very computer can connect.