Port Forwarding and PAT
Learn how mapped ports let outside traffic reach inside hosts.
Sharing One IP With Ports
The most common form of NAT is PAT (Port Address Translation), also called NAT overload. It lets hundreds of devices share a single public IP address by using port numbers to tell their conversations apart.
This is what your home router does: every device behind it reaches the internet through one public address, distinguished only by port.
How Ports Identify Flows
Every TCP or UDP connection has a source port and a destination port. PAT assigns each internal connection a unique source port on the public address, so each flow is uniquely identified by the combination of public IP and port.
When a reply arrives for a given port, the router looks up its table to find which internal device and port it belongs to, then delivers it correctly.
All lessons in this course
- How NAT Shares One Public IP
- Port Forwarding and PAT
- Time and Naming Services
- Proxies and Load Balancers