Router Firewall 

All router firewall devices are basically hardware firewalls. They keep people and software on the outside of your network from getting in without being invited. This includes people attempting a port scan of your system. Router firewalls are a simple solution for connection sharing and basic network protection.  Also see Windows XP Firewall
 
Routers with a protocol called NAT or Network address translation and are often called a nat firewall. Nat allows multiple computers to access the internet through a single internet connection and single IP address. The computers on the inside of the network, known as an intranet, are given IP address that are non-routable (that is a fancy term meaning non-internet addresses). The router translates the requests from your computer into requests from the single IP address on the WAN or Internet side of the router. The router will NOT encode requests originating from the outside of the network and pass them on to the local network unless very specifically configured to do so. In this way, the hardware router acts as a router firewall.  Verify this with a firewall test
 
 
Some router firewalls also have wireless access points built in. They handle the translation between three networks – the wireless network, the wired local network and the internet or WAN network. Wireless computers are local wired computers are ‘bridged’ together behind a firewall by the router. That means wireless computers are free to talk to wired computers without any limitations.  The blue line below represents the firewall

 

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