Port Address Translation (PAT) - Learn Tech From Zero

2.1 Explain characteristics of routing technologie

Address Translation

📘CompTIA Network+ (N10-009)

1. What is PAT?

Definition: PAT is a type of Network Address Translation (NAT) that allows multiple devices on a private network to share a single public IP address when accessing the internet.
Also called: NAT overload.
Key difference from basic NAT: While regular NAT maps one private IP to one public IP, PAT maps multiple private IPs to a single public IP by using port numbers.

2. How PAT Works

Each device on a private network has a private IP address (like 192.168.1.10).
When a device sends traffic to the internet, PAT changes the source IP address to the router’s public IP address.
To keep track of each session, PAT uses port numbers. Each outgoing connection is assigned a unique port number.

Example (IT environment style):

Suppose a company has a single public IP: 203.0.113.5.
Three internal computers want to access a website:
- PC1: 192.168.1.10 → assigned port 50001
- PC2: 192.168.1.11 → assigned port 50002
- PC3: 192.168.1.12 → assigned port 50003
The website sees all traffic coming from 203.0.113.5 but the router keeps track of which port belongs to which internal device.
When the website responds, the router checks the port number and sends the data back to the correct internal computer.

3. Why PAT is Important

Saves public IP addresses: Many internal devices can use one public IP.
Improves security: External users only see the public IP, not internal private IPs.
Supports multiple connections: Thousands of internal devices can access the internet simultaneously using different port numbers.

4. Key Concepts to Remember for the Exam

Port numbers are crucial. PAT relies on TCP/UDP port numbers to distinguish connections.
Single public IP: PAT typically uses one public IP for many internal devices.
Dynamic mapping: PAT dynamically assigns ports for each outgoing connection.
TCP vs UDP: Works with both TCP and UDP connections.

5. Example in an IT Network

Imagine an office with:

50 computers
Only 1 public IP from the ISP

Without PAT: Only 1 computer could access the internet at a time.
With PAT: All 50 computers can access the internet simultaneously. The router keeps track of which responses go to which computer using ports.

Incoming request from internet: PAT usually blocks unsolicited incoming traffic unless a port forwarding rule is set.
Outgoing request to internet: PAT automatically assigns a unique port for each connection.

6. Exam Tips

PAT is a type of NAT. Know the difference:
- Static NAT: 1:1 mapping (private → public)
- Dynamic NAT: multiple private IPs → pool of public IPs
- PAT (NAT Overload): many private IPs → 1 public IP using ports
Understand why PAT is used: IP conservation and security.
Remember that port numbers are used to distinguish sessions.

✅ Quick Summary Table:

Feature	PAT (NAT Overload)
Private to Public IP Mapping	Many → One (using ports)
Port Numbers	Used to track multiple sessions
Public IP Usage	Single public IP can handle many devices
Security	Hides internal IPs
Best For	Offices or networks with limited public IPs

PAT is an essential topic for the Network+ exam, because it combines NAT, IP conservation, and security—all core networking concepts. Understanding the role of port numbers is crucial to pass questions about PAT.