PAT and NAT
Each computer on the network is assigned an IP address. If that address is static, it is relatively easy for a hacker to find it and gain access to the computer more than once. With a static, reliable IP address, a hacker can use a computer as a staging area for launching long, sustained attacks.
Port Address Translation (PAT) and Network Address Translation (NAT) are addressing methods that make internal network addresses invisible to outside computers. PAT and NAT hide the TCP/IP information of hosts in the network being protected to prevent attackers from getting the address of an actual host on your internal network.
PAT and NAT function as a Network-level proxy; the proxy acts as a single host that makes requests on behalf of all internal hosts on the network. NAT hides the identity of hosts from anyone outside of the network by converting the IP address of internal hosts to the IP address of the firewall. To someone on the Internet or outside network, it seems like all information is coming from a single computer.
Application Layer Gateways
Another type of firewall protection is the Application layer gateway, also known as a proxy server. This type of gateway works at the Application layer, the top layer of the OSI model of network communications.
A complete overview of firewalls and what they do would not be complete without mentioning the following security techniques:
- Load balancing
- IP address mapping
- Filtering content
- URL filtering
Firewall Categories
Firewalls can be categorized by processing mode, generation, or structure. Firewalls categorized by level of technology are identified by generation, with the later generations being more complex and more recently developed. Firewalls categorized by intended structure are typically divided into categories including residential-, or commercial-grade, hardware-based, software-based, or appliance-based devices.
Processing Mode
The processing modes are packet filtering, application gateways, circuit gateways, MAC layer firewalls, and hybrids.

Whether you're preparing for Cybersecurity certification, working with government standards, or simply starting your career in compliance, these are the NIST Federal Information Processing Standards (FIPS), Special Publication (SP), and Interagency Report (IR) topics