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For multiple external IP, if you're talking about Public IPs it would depend on your ISP subscription. You can check the subnet mask given to you by your ISP and from there you can determine the IP Address range. Whether you have multiple IPs or not.
That article is ok; my only beef with it is that it doesn't cover established and related connections. Configuring a firewall is greatly simplified if you don't need to worry about return traffic that was "established" by an allowed rule. Here is an iptables tutorial which covers that bit to help supplement your suggestion. https://wiki.centos.org/HowTos/Network/IPTables
If you are running an internet server, you may need to understand iptables, but even that isn't always true.
If you are talking about a desktop or laptop, it depends on how you connect to the internet. A router should have its own firewall, but if you use a modem (which includes a mobile-phone company dongle), you need a firewall on the computer.
Some distros have it set up, but those derived from Debian generally don't. If you have something like Mint or Ubuntu (you really do need to tell us your distro when asking a question!) then
1. open a terminal window
2. enter gufw
3. if it says that it can't find gufw, then install it with sudo apt-get install gufw and then use it.
4. gufw will open a window with a box labeled "status off". Click on that, give your password, and wait patiently until it changes to "status on". That's it!