What Is Port Address Translation (PAT)?
An IP address distinguishes each device connected to a network. Without this unique identifier, modern computer networks and the Internet as we know it wouldn't function. Therefore, using IP addresses efficiently while maintaining robust communication capabilities really matters. One technology that significantly contributes to this goal is port address translation (PAT).
PAT is used in networks to manage the IP address space efficiently. In this article, learn what port address translation is, how it works, and its benefits in networking.
What is port address translation?
Port address translation (PAT) is a subset of network address translation (NAT) that allows multiple devices on a private network to be mapped to a single public IP address but with a different port number identifying each session.
This mapping lets multiple users within a local area network (LAN) make use of a minimal number of IP addresses. It is an addition of network address translation.
To understand PAT, it's essential to be familiar with the concept of NAT. NAT is a method routers use to translate private, not globally routable IP addresses within a network into a single public IP address.
This translation is crucial because the number of available IPv4 addresses is limited, and not all devices require a public IP address to communicate with the Internet.
PAT takes this a step further by allowing multiple devices to share a single public IP address simultaneously. It assigns a unique port number to each outgoing connection.
When a device within the LAN requests to access the Internet, the router assigns a port number to that connection. The router then uses the combination of the public IP address and the unique port number to determine which device it should send the incoming packets to.
PAT works in both small-scale and large-scale networks. Its importance becomes increasingly evident in environments where numerous devices need Internet access.
Allowing multiple devices to share a single local IP address maximizes the utility of available IP resources - a critical function given the scarcity of IPv4 addresses.
How does PAT work?
When an internal device sends a packet to an external network, the router's PAT function modifies the packet. The source IP address is changed from the private IP to the public IP, and the source port number is altered to a unique value.
The router maintains a PAT table that maps the internal private IP addresses and port numbers to the modified source port numbers. This table ensures that when the response comes back from the external network, the router can direct it to the correct internal device.
For example, in a typical home network, multiple devices use the same public IP address to access the Internet. Port address translation assigns a unique port number to each connection established by a device on the internal network. This means different devices can all be online at the same time without any conflicts.
When a device wants to connect to something on the Internet, like a website, the router gives it a specific port number.
The router keeps track of which device is using which port number in a table. When the Internet sends back information, the router uses the table to send the right data to the right device based on the port number it used.
Routers leverage either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port numbers to implement port address translation.
PAT uses 16-bit port numbers to allow for a theoretical maximum of 65,536 unique ports per external IP address. While this theoretical limit isn't always achievable in practice, it still enables a significant number of internal devices to share a single public IP.
Benefits of PAT in networking
The role of PAT in network management is noticeable. You can use it to:
- Conserve public IP addresses. Port address translation is a scalable solution for connecting multiple devices to the Internet. In scenarios where public IP addresses are scarce, the use of port numbers enables efficient utilization of available resources. It also helps in the conservation of valuable IPv4 addresses.
- Simplified network setup. Configuring a network with PAT is generally simpler than managing individual public IP addresses for each device. The administrator doesn’t need to track individual public IP addresses.
- Lessen security flaws. PAT obscures the internal IP addresses of devices on a network. In this case, only the public IP address and different port numbers are visible to the external network. Potential attackers find it challenging to target specific devices within the LAN.
- Improve performance. This network feature allows devices on a private network to share a single public IP address. As such, there’s a reduction in the number of routing entries in the routing table of the router. This contributes to the efficiency of Internet routing.
- Cost saving. PAT is beneficial for organizations that cannot afford a unique public IP address for every device or for those who wish to hide the complexity and details of their internal network from the outside world.
- Control over a network. With port address translation, you can easily add new devices to your network or remove existing ones without affecting the router's public IP address.
For networks that need to conserve IPs, maintain a simple setup, or simply save money on protecting an internal network, port address translation provides an accessible and simplified solution.
Limitations of PAT
While PAT offers several benefits for network management, it also comes with a set of disadvantages that can impact network functionality. It creates complexity in hosting services, complicating the hosting of services that require inbound connections like web or email servers. It's primarily designed for outbound connections.
PAT also has a theoretical limit to the number of supported simultaneous connections due to the finite number of available ports.
Furthermore, when both sides of a communication use PAT, the communication loses end-to-end traceability. This can make it difficult to track individual sessions, complicating network troubleshooting.
Types of PAT
There are mainly two types of PAT: static PAT and overloaded PAT.
Static PAT involves a one-to-one mapping between a private IP address and a public IP address, with a designated port number for each mapping. Hosting services that require inbound connections, such as the web, typically utilize it.
Also known as NAT overload, overloaded PAT allows multiple private IP addresses to share a single public IP address by using unique port numbers for each session.
It's the most common form of port address translation used to conserve the limited IPv4 address space while allowing multiple devices to access the Internet.
We implement these types of PAT based on the requirements of the network. While static PAT is more suitable for specific services, overloaded PAT offers greater flexibility and efficiency for general Internet access for multiple users.
NAT vs. PAT
PAT (port address translation) and NAT (network address translation) are related concepts, but they aren't the same. Consult the table below to understand the differences between NAT and PAT.
| Feature | NAT | PAT |
| Full form | Network Address Translation | Port Address Translation |
| Basic function | Translates private IP addresses to a single public IP address | Extends NAT by translating IP addresses into a single public address via port number |
| IP address translation | Translates only the IP addresses between private and public networks | Translates both IP addresses and port numbers between private and public networks |
| Device identification | Devices are identified by their private IP addresses | A combination of private IP and port numbers identifies devices |
| Port number assignment | Does not change port numbers; multiple devices use the same port | Assigns a unique port number to each device's session |
| Scalability | Low, due to limited port availability | High, because it uses port numbers to differentiate sessions |
| Overhead | Lower | Higher due to port mapping |
When to use network address translation
You can use NAT to:
- Map a single private IP address to a single public IP address.
- Isolate the internal network from the external network.
- Avoid IP address conflict between private subnets from two different parties.
You can also use NAT when the same IP range is used in different internal networks that need to communicate.
When to use port address translation
Use PAT when:
- You have multiple devices within your private network that all need Internet access, but you only have one public IP address from your Internet service provider.
- More devices need to simultaneously access the Internet.
- You want to target a simpler network set up that’s easy to manage.
The widespread adoption of IPv6 could eventually reduce the need for NAT and PAT. However, these technologies will likely remain relevant for the foreseeable future as the transition to IPv6 continues.
Frequently asked questions
Can I use PAT with IPv6?
PAT is primarily designed for IPv4. It is not typically necessary for IPv6 because IPv6 has a much larger address space that eliminates the need for address conservation techniques like PAT.
Is PAT compatible with all network devices and systems?
PAT can be used with most network devices and systems, especially since it's a common feature in consumer-grade routers. But it's always best to check the specs of a device because not all may support PAT.
Does port address translation affect network performance?
PAT can affect it because it adds an extra layer of processing. The translation of IP addresses and port numbers requires additional CPU and memory resources, which can potentially impair throughput in high-traffic scenarios
Can PAT cause issues with certain applications?
Yes, it causes issues with applications, particularly those that are sensitive to changes in IP addresses and port numbers.
