NAT (Network Address Translation) is a method of remapping one IP address space into another. It is primarily used to allow devices on a private network to communicate with the public Internet.
Key Benefit
NAT conserves the limited supply of public IPv4 addresses by allowing thousands of internal devices to share a few public IPs.
Types of NAT
1. Static NAT
One-to-One mapping. Used for servers (Web, Mail, DNS) that need to be consistently accessible from the outside world using the same public IP.
2. Dynamic NAT
Many-to-Many mapping. Maps a private IP to a public IP from a pre-defined pool of available addresses. If the pool is empty, new connections are blocked.
3. PAT (Port Address Translation)
NAT Overload: Many-to-One mapping. Maps multiple private IPs to a single public IP by assigning different source port numbers to each connection. This is the industry default for home and enterprise networks.
NAT Terminology
| Term | Description |
|---|---|
| Inside Local | The private IP address of the device on the internal network. |
| Inside Global | The public IP address the internal device uses to reach the internet. |
| Outside Global | The public IP address of the destination device on the internet. |
When configuring NAT on Cisco routers, always remember to define `ip nat inside` and `ip nat outside` on the correct interfaces, or translation will never trigger!