Countermeasures - Firewalls

Firewalls are used to keep networks secure by analyzing data packets and determining whether they should be allowed or not. The following are types of firewalls:

• Packet filters - Looks at each packet and decides whether to allow it or not; Most routers already have packet filters implemented.
• Network Address Translation (NAT) - Hides entire local network behind one IP address; Makes an entire network look like a single machine.
• Proxies - Prevent you from actually connecting to the internet; Proxies never connect to the URL, but instead connect to the proxy server, and the proxy server gets the URL for you. 

image source: http://www.howstuffworks.com/firewall.htm