Super noob trap, which this article doesn't address. If you want to scan every port, do -p-. Which alot of the times you do want to scan every port. By default it only scans common ports. You can also do -p50- to scan 50+ or -p-50 to scan 0-50. You can find alot on non common ports, and if you're running a port scan, you should use it. Sad the article doesn't mention this.