PAT in Networking || Port Address Translation

Introduction

Definition: PAT stands for Port Address Translation. It is a type of NAT that lets many devices in a private network share one public IP address to access the internet. It works by translating not only the private IP address, but also the source port number. That port change is what makes sharing possible.

PAT is everywhere. From Wi-Fi routers to enterprise internet gateways, all use it. Service providers use a larger version of it called CGNAT. If you work in networking, you will see PAT in day-to-day troubleshooting more often than you expect.

This guide will explain what PAT is, its types, how it works, and why it is important for networking.

What is Port Address Translation in Networking?

PAT (Port Address Translation) is a type of NAT that lets multiple private devices share one public IP by translating the source ports, so return traffic reaches the correct internal host.

PAT takes multiple inside private sessions and maps them to one public IP by using different source ports. So instead of doing a pure one-to-one IP translation, it does a many-to-one translation. One public IP can represent thousands of internal sessions because each session gets a unique port on the public side.

Devices that perform port address translation work on layer 3 of the ISO/OSI reference model and are often Internet access routers such as DSL or cable routers. Thanks to PAT, a private network only needs a public IP address to communicate with other devices on the Internet.

Importance of PAT in networking

PAT is important in networking for multiple reasons. Some of these are:

• It saves rare and expensive public IP addresses. You don’t have to buy or rent a public IP address for each device on your network if you use PAT. Instead, many devices on your network can share one public IP address.
• It makes your devices safer because it keeps your secret or private IP addresses from being seen by the internet. Attackers can only see the public IP address of your router, which makes it harder for them to scan or directly target your devices.
• It gives you more control over your network because you can add or remove devices without changing your router’s public IP address. You can also change your devices’ private IP addresses without changing the PAT process.
• It supports session-based traffic. Modern apps open many short connections, and PAT tracks each session using ports so traffic stays separated even on one public IP.

How Port Address Translation Works?

PAT generates a database i.e., a mapping table that stores which private IP address and port number are associated with which public IP address and port number. The router is where the mapping table is kept when connecting a private network to the wider internet. The router performs the translation of the IP addresses and port numbers into their corresponding formats using the mapping table.

Let’s understand with the help of an example.

Think of a home network that includes your computer, your smartphone, and your smart TV. The router’s public IP address is 203.0.113.1, and it gives out the following private IPs to the connected devices:

• Now the computer tries to access a website with an IP address of 198.51.100.1. It sends a request to the router.
• In this case, the router receives the IP packet from the computer and performs PAT on it. The router then generates an entry in the mapping table, giving the computer a port number, say 1000.

After then, the router changes the computer’s private IP address and port number to the network’s public IP address and port number before sending the request over the internet. At this stage the router then sends the modified IP packet over the internet.

• The remote host recognizes the client’s IP address and port number as 203.0.113.1:1000 and processes the request accordingly.
• After receiving the response, the router analyzes its mapping table to determine the source of the request.
• At this stage the router receives the response from the destination server and performs reverse PAT on it. When a response is received, the router translates the source’s public IP address and port number back into the original source computer’s private IP address and port number before sending the data back.

Note: The same process is repeated for the other devices on the home network, such as the smartphone and the smart TV, but with different port numbers assigned by the router.

That’s how PAT works.

How PAT increases Network Efficiency?

PAT can allow hundreds or thousands of devices to share a single public IP address, thus reducing the demand for public IP addresses. This is very valuable for networks dependent on IPv4, as the public addresses are limited there. So, PAT increases the network efficiency by allowing many devices to use the same public IP address.

Different Types of Port Address Translation

Mainly there are two types of PAT in networking. These are:

Static PAT

Static PAT is a type of PAT that allows a device on a private network to have a fixed port number for a specific service or application. Let’s take an example for better understanding.

If a device wants to host a web server on port 80, it can use static PAT to map its private IP address and port 80 to a specific public IP address and port number, say 203.0.113.1:8080.

Static PAT is useful for allowing external users to access services or applications hosted on devices on a private network.

Overloaded PAT (NAT overload)

PAT is also called NAT overload. Overload means one public IP is overloaded with many internal sessions, separated by ports. You will see this term in configs because many vendors name the feature that way.

Let’s understand in detail with an example.

If two devices want to access different websites using port 80, they can use overloaded PAT to map their private IP addresses and port 80 to the same public IP address and port number, say 203.0.113.1:1000.

Overloaded PAT is useful for maximizing the use of available public IP addresses and port numbers.

Difference between NAT and PAT

The basic difference between NAT and PAT is:

How to Configure Port Address Translation?

PAT can be configured in both simple and complex networks. Let’s see these both one by one:

Basic PAT Configuration

Step 1: Configure the router to use a single Public IP Address.

Step 2: Assign private IP address to internal devices.

Step 3: Enable PAT to map private IPs to the public IP using different port numbers.

Advanced PAT Configuration

For complex networks, advanced PAT settings can include:

• Specific Port Ranges.
• Priortizing certain traffic.
• Handling large number of connections simultaneously.

Advantages of Port Address Translation

Here are a few advantages of PAT in networking:

• Efficient use of IP Addresses: Port Address Translation allows multiple devices to access the Internet using a single IP Address, reducing the demand for IPv4 addresses.
• Enhanced Security: It adds a layer of security as the IP addresses of internal devices are hidden from the public internet.
• Cost-effective: It is cost-effective as it allows organizations and ISPs to use the same public IP, instead of multiple public IPs.
• Seamless Internet Sharing: It allows multiple users and devices in homes, offices, and businesses to use the Internet at the same time.
• Reduces complexity at the edge: Instead of routing public addresses deep into the LAN, you keep public routing at the edge device.

Limitations of Port Address Translation

Despite having so many advantages, there are a few disadvantages associated with Port Address Translation. Some of these are:

1. Limited Number of Ports: PAT relies on port numbers ranging from 0-65535 for translation, so, a single IP can handle a limited number of simultaneous connections.
2. Incompatibility: There are several applications such as VoIP, and peer-to-peer (P2P) services, that struggle with PAT as they rely on direct IP communication.
3. Increased Processing Overhead: Routers handling PAT need increased memory and CPU usage as they have to maintain a table of active translations.
4. Port conflict: Since so many devices use the same public IP, two or more devices might try to use the same port number, which can lead to a conflict.
5. Complexity: It introduces complexity into the network configuration and troubleshooting, as each connection is tracked by a unique port number.
6. Inbound access needs explicit configuration: If you want inbound connections, you must use static PAT or a firewall policy. Otherwise the router has no idea which internal host should receive the traffic.
7. Asymmetric routing breaks NAT: NAT and PAT assume return traffic comes back through the same device that created the translation. If traffic returns through a different path, the mapping is missing and sessions fail.
    

Frequently Asked Questions

Q1. What is PAT in networking?

PAT is an extended version of NAT that helps in mapping multiple devices to a single public IP address in order to preserve IP addresses.

Q2. What is NAT and PAT in networking?

NAT stands for network address translation, which is used to translate private IP addresses to public IP addresses. PAT stands for port address translation, which is used to translate private IP addresses to public IP addresses via port numbers.

Q3. What is the PAT used for?

PAT is used for security and efficiency purposes, as it can hide the internal network structure and reduce the demand for IP addresses.

Q4. Why use pat instead of NAT?

Unlike NAT, PAT uses source port numbers, allowing for several hosts to share a single IP address while each utilizing a unique port.

Conclusion

“Port Address Translation also known as PAT is a networking technique that allows various devices inside a private network to use a single public IP address by using distinct port numbers.” It works by translating source ports and maintaining a session mapping table. If you understand how the mapping is created, how return traffic matches it, and where it breaks, you can troubleshoot NAT issues much faster in real networks. It proves to be a valuable tool in the preservation of IPv4 addresses and enhancement of security measures by means of hiding private IP addresses associated with devices from external internet access. In this blog post, we have learned what is PAT in networking, the different types, how it works, and the benefits that it brings to networking.

If you are interested in learning more about NAT and other networking concepts, it’s best to learn about these things in a hands-on, practical way. One way to start is to take CCNA Course.