Understanding Network Address Translation (NAT)

Understanding Network Address Translation (NAT)

Network Address Translation (NAT) is a method used to map private IP addresses to a single public IP address. It allows multiple devices in a local network to access the internet using a shared public IP. But before exploring the NAT, Let’s see what is private and public network addresses.

Private IP Addresses in Networking

In networking, private addresses are reserved IP addresses that are not routable on the public internet. They are designated for use within private networks, allowing multiple devices to communicate internally without requiring unique public IP addresses. The three commonly used private IP address ranges are:

Class A Private Address Range:

  • Example: to
  • Devices within a network using IP addresses in this range can communicate with each other privately.

Class B Private Address Range:

  • Example: to
  • Similar to Class A, devices within this range can communicate privately within their network.

Class C Private Address Range:

  • Example: to
  • Devices using addresses in this range are commonly found in home networks and small businesses.

Public IP Addresses in Networking

Public IP addresses are unique numerical labels assigned to devices on a network that are directly accessible from the internet. These addresses are globally routable and serve as the public-facing identifiers for devices or servers connected to the World Wide Web.

Why use NAT?

IP addresses are limited, and NAT helps conserve public IP addresses by allowing multiple devices to use a single public IP. It also adds a layer of security as internal IPs are not directly exposed to the internet.

Types of NAT

Static NAT: Static NAT involves a one-to-one mapping of private IP addresses to corresponding public IP addresses. This type of NAT is particularly useful when specific services or devices within a private network need to have a consistent, dedicated public IP address. For example, if a web server inside the network requires a fixed public IP for external access, static NAT would be employed.

  • Scenario: Imagine a company with an internal web server having a private IP address of To make this web server accessible from the internet with a consistent public IP, a static NAT mapping could be set up.
  • Configuration: Static NAT mapping could be configured as follows:
  • Private IP: Public IP:

Dynamic NAT: Dynamic NAT allows multiple private IP addresses to be mapped to a pool of public IP addresses dynamically. This means that devices within the private network are assigned a public IP address from the pool on a temporary basis. Dynamic NAT is more scalable than static NAT, as it doesn’t require a dedicated public IP for each internal device. However, it may lead to less predictable mappings.

  • Scenario: In a university with numerous students accessing the internet, dynamic NAT can be used. Each student’s device gets a temporary public IP from a pool when accessing online resources.
  • Configuration: Dynamic NAT pool configuration might look like this:
  • Private IP Range: – Public IP Pool: –

PAT (Port Address Translation): Port Address Translation, also known as PAT, is a form of dynamic NAT where multiple private IP addresses are mapped to a single public IP address. The distinction is made by using different source port numbers for each connection. This allows multiple devices within the private network to share the same public IP address while being uniquely identified by the combination of IP address and port number. PAT is the most common form of NAT and is often used in home and small business networks.

  • Scenario: In a small office where multiple employees use the internet, PAT can be employed to share a single public IP. Each device is identified by its unique combination of private IP and source port.
  • Configuration: PAT might use the router’s public IP ( with a different source port
  • Router’s Public IP:
  • Private IP 1:, Source Port: 5001
  • Private IP 2:, Source Port: 5002

Overloading NAT: Overloading NAT is another term for PAT, emphasizing the practice of “overloading” a single public IP address with many private IP addresses using different port numbers. This method maximizes the utilization of a limited number of public IP addresses and is a crucial aspect of addressing the scarcity of available IPv4 addresses.

  • Scenario: A home network with several devices sharing one public IP address through PAT (overloading NAT). Devices are distinguished by different source port numbers.
  • Configuration: Overloading NAT uses the router’s public IP ( and assigns unique source ports:
  • Router’s Public IP:
  • Private IP 1:, Source Port: 6001
  • Private IP 2:, Source Port: 6002

Network and power connections at the rear
Network and power connections at the rear by U.S. Department of Energy is licensed under CC-CC0 1.0

NAT Operation Overview

NAT configuration is typically implemented on a gateway router or border device. This device has two interfaces: one connected to the local area network (INSIDE) and another connected to the external network (OUTSIDE).

Device Configuration: The NAT device is set up with a configuration pool to manage the translation of private IP addresses to public IP addresses and vice versa.

Local to Public Translation: When a request originates from a local machine within the internal network (INSIDE), it first encounters the NAT configuration. The private IP address of the local machine is then translated into a public IP address from the configured pool. This translation allows the local machine to communicate with external hosts on the internet using the shared public IP.

Address Spaces:

Inside Local Address: Represents the IP address of a device within the local network, as seen from within the same network.

Outside Global Address: The IP address of the external target host as seen from the external network, before translation.

Bidirectional Translation: NAT operates bidirectionally, translating private to public addresses for outgoing traffic and public to private addresses for incoming traffic. This bidirectional translation ensures seamless communication between devices inside the local network and external hosts on the internet.

Self Assessment

  • What are the primary types of Network Address Translation (NAT), and how do they differ in terms of address mapping? Provide a brief explanation for each.
  • Explain the difference between Dynamic NAT and PAT (Port Address Translation). When would you choose one over the other in a networking scenario?
  • Define the terms “Inside Local Address,” “Outside Local Address,” and “Outside Global Address” in the context of NAT. How are these address spaces used during the translation process?
  • Describe the bidirectional nature of NAT. How does NAT facilitate the translation of private to public addresses for outgoing traffic and public to private addresses for incoming traffic?

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