LOL, while I am posting this my grandson is watching SpongeBob on TV.
All Entry Level Cable/DSL Routers by default are NAT-Firewall. Some have additional protection like Statefull Package Inspection (SPI).
When you use few computers to share one Internet connection, the information that comes from the Internet needs to know to which computer it belongs. The main function of Cable/DSL Router is to Route the Internet signal to the requesting computer.
This function is called Network Address Translation (NAT).
As result information that comes from the Internet and was not requested by one of you LAN?s computers (e.g. hacking attempts) does not know where to go, and it is blocked, hence NAT Firewall.
When you are connected to a Website or downloading files, a lot of junk can get to your Hard Drive. Since you requested the pages for the site the NAT Firewall will not block what comes in from this page. If the ?Site Keeper? loaded the page with ?Junk? it will get to your computer.So you have NAT-Firewall but you can end up with Viruses, Zombies, Trojan etc. "Dished" to you by sites that you visited on your own volition,
Since the NAT-Firewall blocks ony Incoming, any communication initiated from any of your LAN?s computers will go out to the Internet, and will be answered. As a result
programs calling home, spywares, ?zombies? etc. can communicate freely in lieu of the NAT Firewall. You will not be aware of these activities unless you monitor the communication locally with a software Firewall.
Most decent software Firewalls will alert you to existence of this communication, and will allow controlling it in various ways.
In addition using programs like Ad-ware, and Trojan Blocker, will further secure the Network.
How much safety you need? It is a matter of Surfing habits and personal preference.
Link:
Basic Protection for Broadband Internet Installation.
Link:
What is the Best Firewall?
