What is Network Address Translation (NAT)?

Network Address Translation (NAT) is a networking technology used to modify network address information in packet headers while in transit through a router or firewall. NAT plays a crucial role in enabling multiple devices on a local network to share a single public IP address for communication with external networks, such as the internet. 

Dissecting Network Address Translation (NAT)

Network Address Translation (NAT) emerged in the early 1990s as a solution to the imminent depletion of available IPv4 addresses. As the Internet expanded rapidly, the pool of public IPv4 addresses dwindled. IPv4, the dominant internet protocol, employs a 32-bit address space, allowing approximately 4.3 billion unique addresses. However, with the exponential growth of internet-connected devices, it became clear that this address space would soon be exhausted. NAT was introduced to address this issue and extend the usability of IPv4 by enabling more efficient utilization of existing addresses.

How NAT works

To enable multiple devices on the local network to share a single public IP address while providing security by concealing the internal network structure, NAT dynamically modifies the source and destination IP addresses and port numbers in packet headers as data flows between the local network and external networks.

1. Packet Arrival: When a device on the local network initiates communication with an external destination (e.g., a web server on the internet), it generates a data packet. The packet contains the source IP address (the private IP of the local device), source port number, destination IP address (the external destination's public IP), and destination port number (commonly associated with a specific service, like HTTP on port 80).
2. NAT Device Inspection: The data packet is routed through the local network's NAT device, typically a router or firewall. The NAT device inspects the packet header to determine if it is subject to NAT translation.
3. Translation Rules: If the packet meets the criteria for NAT translation (e.g., it is outgoing traffic from a local device), the NAT device applies a translation rule from its NAT table.
4. Source NAT (SNAT) - Outbound Traffic: In the case of outgoing traffic, the NAT device performs Source Network Address Translation (SNAT), also known as Outbound NAT. The NAT device replaces the source IP address in the packet header (the private IP of the local device) with its own public IP address. This allows the packet to be routed on the internet using the NAT device's public IP address. The NAT device also assigns a unique source port number (often from a pool of available ports) to the packet to maintain the distinction between different local devices sharing the same public IP address. The translated packet is then forwarded to the external destination.
5. Destination NAT (DNAT) - Inbound Traffic: When a response packet is received from the external destination, it is directed to the NAT device because the destination IP address in the packet header is the public IP address of the NAT device. The NAT device uses its NAT table to perform Destination Network Address Translation (DNAT), also known as Inbound NAT. It examines the destination port number in the packet header to determine which internal device the response packet is intended for. The NAT device replaces the destination IP address in the packet header (its own public IP) with the private IP address of the specific internal device and the corresponding port number. The translated response packet is then forwarded to the appropriate internal device.
6. Stateful NAT: NAT devices maintain state information in their NAT tables to keep track of ongoing connections. This allows them to correctly map incoming response packets to the appropriate internal device.

Types of NAT

There are several types of Network Address Translation (NAT), each designed to serve different purposes in network configuration. The main types of NAT include:

• Static NAT (SNAT): Static NAT is a one-to-one mapping of private IP addresses to public IP addresses. Each private IP address is statically associated with a specific public IP address. Static NAT is typically used when an organization needs to expose specific internal resources, like web servers or FTP servers, to the public internet.
• Dynamic NAT (DNAT): Dynamic NAT maps a range of private IP addresses to a pool of public IP addresses. It dynamically assigns a public IP address from the pool for outgoing traffic requests on a first-come, first-served basis. This type of NAT is suitable for situations where a limited pool of public IP addresses needs to be shared among multiple internal devices.
• Network Address Port Translation (NAPT) or Port Address Translation (PAT): NAPT, also known as PAT, is a type of NAT that maps multiple private IP addresses to a single public IP address. It differentiates between internal devices using unique port numbers. NAPT is commonly used in home and small office routers to allow multiple devices to share a single public IP address. Each internal device is assigned a unique port number, which allows for the translation of multiple internal IPs to a single public IP.
• Overloading NAT: Overloading NAT is a specific implementation of NAPT where the NAT device assigns a unique source port number for each internal device's outgoing traffic. This allows for a large number of internal devices to share a single public IP address. Overloading NAT is often used by ISPs to conserve public IP addresses.
• Cone NAT (Full Cone, Restricted Cone, Port Restricted Cone): These NAT types are characterized by how they handle incoming traffic initiated from external sources. Full Cone NAT allows any external host to initiate a connection to an internal host using the translated IP and port. Restricted Cone NAT only allows incoming traffic from an external source if the internal device has previously initiated communication with that source. Port Restricted Cone NAT further restricts incoming traffic to the specific port number used in the outgoing connection.
• Symmetric NAT (Symmetric NAT Mapping): Symmetric NAT is a type of NAT that assigns a unique mapping for each internal device for both the source IP address and source port number. This means that each outgoing connection from an internal device appears as a unique external IP and port combination. Symmetric NAT can be a challenge for peer-to-peer applications and can require techniques like STUN (Session Traversal Utilities for NAT) and TURN (Traversal Using Relay NAT) for successful communication.