NAT

How NAT Works

A NAT device — typically a home router or firewall — maintains a translation table mapping (private IP, private port) tuples to (public IP, public port) entries. When an internal host at Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication. 192.168.1.50 connects to a web server, the router replaces the source address with its Public IP AddressA globally unique IP address assigned by an ISP that is routable on the public internet. Every device directly accessible from the internet must have a public IP address. and records the mapping. When the web server responds, the router looks up the destination port in its table and rewrites the packet back to 192.168.1.50, then forwards it inward.

This process is called NAPT (Network Address Port Translation) or PAT (Port Address Translation) — the most common form where thousands of internal sessions share a single public address, distinguished by port numbers.

NAT Types and Traversal

NAT behavior varies significantly. Full cone NAT maps a private endpoint to a fixed public endpoint, allowing any external host to send packets to the mapped port. Symmetric NAT creates a new mapping per destination, making peer-to-peer connections difficult. STUN, TURN, and ICE protocols — used by WebRTC and VoIP — exist primarily to traverse NAT, discovering the public endpoint and punching holes through firewalls.

NAT's Impact on the Internet

NAT was designed as a stopgap for IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. exhaustion, not an architectural feature. It breaks the internet's end-to-end connectivity model: hosts behind NAT cannot receive unsolicited inbound connections without port forwarding configuration. Services that require inbound reachability — game servers, VoIP endpoints, peer-to-peer applications — must explicitly configure port forwarding or use relay servers. IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. eliminates this problem by providing every device with a globally routable address.

Check your public IP (the one NAT exposes) with IP Lookup.