- Have some program listen on that port. If there's nothing listening, it's not "open" in any sensible way.
- Make sure no firewall (iptables) rule or policy is blocking the port. No rules/policies at all means nothing is blocked, so in that case it's "open" if there's just something listening on it.
- If you have configured a firewall and it's policies block the port, add a new rule that lets traffic pass trough it. Or if some rule denies it, take that rule out. An example to accept tcp traffic in on port 6000:
/sbin/iptables -A INPUT -p tcp --dport 6000 -j ACCEPT
you could do the same for OUTPUT if you had set output policy to drop. Note: the above command is in use right after you issue it, you need to be root to set it (and have iptables installed in the first place), and after reboot it's lost unless you save it to a script that's run during boot (some use rc.firewall).