How does NAT (Network Address Translation) work?

Network Address Translation (NAT) is a technique used in computer networking to enable multiple devices within a local network to share a single public IP address for communication with devices on the internet. NAT plays a crucial role in conserving IPv4 addresses, as the number of available IPv4 addresses is limited.

Here's a detailed explanation of how NAT works:

1. Local Network Setup:
   • A local network consists of multiple devices (computers, smartphones, printers, etc.) that are assigned private IP addresses. Commonly used private IP address ranges include:
     • 10.0.0.0 to 10.255.255.255 (10.0.0.0/8)
     • 172.16.0.0 to 172.31.255.255 (172.16.0.0/12)
     • 192.168.0.0 to 192.168.255.255 (192.168.0.0/16)
2. Public and Private IP Addresses:
   • Devices within the local network use private IP addresses, which are not routable on the public internet. Public IP addresses, on the other hand, are globally unique and can be used for communication over the internet.
3. Router Configuration:
   • The local network is connected to the internet through a router. The router is configured with both a public IP address for communication with the internet and a private IP address for communication within the local network.
4. Outgoing Data:
   • When a device within the local network wants to communicate with a server on the internet, it sends a data packet to the router.
   • The router modifies the source IP address of the packet from the private IP address of the sending device to its own public IP address.
5. Source Port Translation:
   • To keep track of multiple simultaneous connections, NAT also modifies the source port number of the packet. This process is known as "source port translation" or "PAT" (Port Address Translation).
   • The router maintains a translation table, mapping each private IP address and port to its corresponding public IP address and a unique port number.
6. Routing to the Internet:
   • The router forwards the modified packet to the destination server on the internet.
   • When the server responds, it sends the data back to the public IP address and port from which it received the request.
7. Incoming Data:
   • When the router receives the response packet, it checks its translation table to determine which device in the local network originally initiated the request.
   • The router modifies the destination IP address and port back to the private IP address and port of the corresponding device within the local network.
8. Maintaining State:
   • NAT routers maintain state information to keep track of the ongoing connections. This ensures that the router can correctly translate both outgoing and incoming packets.
9. Conserving IP Addresses:
   • By using a single public IP address for multiple devices within the local network, NAT helps conserve the limited pool of available public IP addresses.

Network Address Translation allows multiple devices within a local network to share a single public IP address for internet communication by modifying the source and destination IP addresses and ports in data packets. This process helps address the scarcity of public IP addresses and provides a level of security by hiding the internal network structure from the outside world.