If you have a VPS do you have a full set of ports that you can access, or is it limited because of other VPS systems on the server using them, causing a clash? Can you specify that you need certain ports?

Each VPS on a dedicated server is like a mini server on its own. You can assign ports as you would on a dedicated.

You can use any ports you wish, which are standard ports in the internet. But of course you might not be able to change the common ports already defined for the services, but you can indeed open any port, and connect any software to run on that port. The only step additional would be to open the port out in the server firewalls.

That's not too bad, but would you have access to the server firewalls? Or would you have to contact the hosting company to have them do it, and then justify why it is that you want a particular port to be opened?

It depends on the company, but most put the firewall under your control. You probably won't even have a firewall unless you install one.

Port numbers are related to the IP address, not the server. Each IP can use port 443 for example. If you have 5 ips, you have 5 x port 443, or 5 ecommerce websites.

Ah, thank you! I was unaware that it was IP linked, I thought it was to do with the physical machine.

Would it be wise to install a firewall? Are the risks large when you open up certain ports?

- Yes, you should always keep your server secure by installing a firewall, for linux VPSs CSF and APF are recommended.
- Keeping ports open can lead to vulnerabilities. Most firewalls block all other ports than the commonly used ones like 80-apache, 25-smtp, 22-ssh.

Or even better get a VPS that's behind a physical firewall!

Make sure that you are sure of the ports you open in the firewall, any unused port opened on any server can be insecure for the VPS.

Absolutely. Keep open the ones which will be used. I see no reason open another ones.

Keeping ports unfiltered have nothing to do vulnerabilities, the services listening on those ports could have a security vulnerability might as well be the httpd or perhaps ftp daemon, it's common practice to keep unused ports filtered to prevent other unwanted daemons to be accessible on the internet such as backdoors/rootkits.

Filtering ports also slow down port scanners (wasn't able to edit my post)