Nat-in

Nat-in is a term primarily used within the field of systems administration and computer networking, specifically referring to a network address translation (NAT) configuration where the internal address of a host is translated to a public address. This is the core function of most NAT implementations. The term "Nat-in" isn't a formal or standardized term defined by RFCs or formal networking documentation, but rather an informal shorthand used to describe the fundamental translation process.

The underlying principle is that a private network, often behind a router or firewall, uses private IP addresses (e.g., 192.168.x.x, 10.x.x.x, 172.16.x.x - 172.31.x.x) that are not routable on the public internet. When a device on the private network initiates a connection to a host on the internet, the NAT device (typically a router) intercepts the outgoing packet. It then modifies the source IP address in the packet header, replacing the private IP address of the originating device with its own public IP address. This process effectively hides the internal IP address from the external network.

The NAT device maintains a translation table that maps the private IP address and port number of the originating device to the public IP address and a chosen port number on the NAT device. When the external host replies, the NAT device consults this translation table to determine the original destination (the internal device) and forwards the packet accordingly, translating the destination IP address back to the internal IP address. This "Nat-in" process enables multiple devices on a private network to share a single public IP address.