Table of Contents
- 1: What is NAT?
- 2: Why Do We Need NAT?
- 3: How Does NAT Work?
- 4: Types of NAT
- 5: Example of PAT (Most Common Form of NAT)
- 6: NAT and Security — How Does NAT Protect Your Network?
- 7: NAT Configuration Basics (Cisco Example)
- 8: NAT vs Firewall — What's the Difference?
- 9: Limitations of NAT
- 11: Conclusion
- Frequently Asked Questions
What is NAT?
Network Address Translation (NAT) is a method used in networking to map private (internal) IP addresses to a public (external) IP address — and vice versa — as data travels between a private network and the internet.
This allows multiple devices within a local network to access the internet using a single public IP address.
Why Do We Need NAT?
- IP Address Conservation- With the limited pool of IPv4 addresses, NAT allows multiple devices to share one public IP
- Network Security- Internal private IP addresses are hidden from the outside world.
- Flexible Network Design- Easily connect private networks to the internet without reconfiguring internal IPs.
How Does NAT Work?
When a device inside a private network sends a request to the internet:
- The NAT device (usually a router) changes the source IP address of the packet to the router's public IP.
- The NAT device keeps a translation table mapping the internal IP and port to the external IP and port.
- The response from the internet is received by the NAT device, which then forwards it to the correct internal device based on the translation table.
Types of NAT
| Type | Description |
|---|---|
| Static NAT | Maps one private IP to one public IP (1:1 mapping). |
| Data VLAN | Carries user-generated data traffic. |
| Dynamic NAT | Maps private IPs to a pool of public IPs (first-come, first-served). |
| PAT (Port Address Translation) | Also called NAT Overload — maps multiple private IPs to a single public IP by using different port numbers. |
Example of PAT (Most Common Form of NAT)
To allow multiple VLANs over a single link (trunk), VLAN tagging is used.
| Private IP | Public IP | Port |
|---|---|---|
| 192.168.1.2 | 203.0.113.1 | 10001 |
| 192.168.1.3 | 203.0.113.1 | 10002 |
This allows many internal devices to share a single public IP without conflict.
NAT and Security — How Does NAT Protect Your Network?
- Hides Internal Network- External users can't directly initiate connections to internal devices.
- Acts as a Basic Firewall- Only allows return traffic for outgoing connections.
- Reduces Attack Surface- Makes internal IP scheme invisible to attackers on the internet.
Note: NAT is not a replacement for a firewall but works as a helpful layer of security.
NAT Configuration Basics (Cisco Example)
ip nat inside source list 1 interface FastEthernet0/0 overload
access-list 1 permit 192.168.1.0 0.0.0.255
- NAT applied on outgoing interface with overload (PAT).
- Access list defines the private network allowed to use NAT.
NAT vs Firewall — What's the Difference?
| NAT | Firewall |
|---|---|
| Translates IP addresses | Controls traffic based on security policies |
| Provides IP masking | Provides detailed traffic filtering |
| Works at Layer 3 (Network) | Works at Layers 3 & 4 (Network & Transport) |
Limitations of NAT
- Breaks end-to-end connectivity (affects some applications).
- Needs port forwarding for hosting services internally.
- Complicates certain protocols like VoIP, which may require NAT traversal techniques.
Conclusion
NAT is a powerful tool for both IP address management and network security. It enables organizations and home users to connect multiple devices to the internet without exposing their internal network — all while making efficient use of limited public IP addresses.
Frequently Asked Questions
Can NAT be used with IPv6?
NAT is generally not required with IPv6 due to its vast address space, but NPT (Network Prefix Translation) is sometimes used.
Is NAT the same as a Firewall?
No. NAT hides IP addresses, while a firewall enforces security policies.
What is Port Forwarding?
It's a NAT feature that forwards traffic from a specific external port to a designated internal IP and port.