PAT (Port Address Translation)

Port Address Translation (PAT) is a specific type of Network Address Translation (NAT) that not only maps IP addresses from one realm to another but also translates the TCP/UDP port numbers of the packets involved in the communication. Often referred to as "Network Address Port Translation (NAPT)," PAT is widely used in both small-scale and enterprise networking environments to allow multiple devices on a local network to share a single public IP address.

PAT works by assigning unique TCP or UDP port numbers to each outgoing connection from a network. This process allows the router to use the same IP address for all outgoing traffic while distinguishing between different sessions based on the translated port number. For example, multiple devices on a home network can access the internet simultaneously through a single IP address provided by the ISP, with the router keeping track of which traffic belongs to which device by using different port numbers.

Here’s how PAT typically functions:

PAT is essential for conserving the limited pool of available IPv4 addresses, making it possible for numerous devices to access the internet with only one public IP address. It also adds a layer of security by masking internal IP addresses from external networks, which can help protect against certain types of network attacks. However, PAT can complicate certain applications and protocols that require end-to-end connectivity or that do not dynamically adjust to port changes, such as some VPN protocols and online gaming or video conferencing services.