Understanding NAT vs PAT in Enterprise Firewall Designs

July 2010    |   Reading time: 10 min

Network Address Translation (NAT) and Port Address Translation (PAT) are foundational technologies in enterprise firewall configurations. Despite their similarities, understanding their key differences is essential for proper policy design, traffic flow control, and service publishing.

What is NAT?

NAT translates one IP address into another. In a typical enterprise setup, it allows internal IP addresses to be mapped to publicly routable ones. This ensures address space conservation and a layer of security through obfuscation. NAT can be static or dynamic, with specific one-to-one mappings.

What is PAT?

PAT extends NAT by allowing multiple internal hosts to share a single external IP address, differentiating sessions via port numbers. This is commonly used for outbound Internet access where many clients initiate connections simultaneously.

Deployment Scenarios

In branch environments, PAT is used to grant Internet access to users behind a firewall. In data centers, NAT is applied to publish internal servers externally, often using static NAT rules. When multiple services must share one IP, PAT rules with port remapping are configured.

Configuration Examples (Cisco ASA)

object network INTERNAL-NET
 subnet 10.1.1.0 255.255.255.0
 nat (inside,outside) dynamic interface
!
object network WEB-SERVER
 host 10.1.1.100
 nat (inside,outside) static 203.0.113.50
  

Security Considerations

NAT and PAT obscure internal addresses but are not security mechanisms by themselves. ACLs and stateful inspection must complement translation policies. Static NAT rules should be tightly scoped, and port-forwarding configurations must avoid exposing unnecessary services.

Troubleshooting Tools

show xlate
show nat
packet-tracer input inside tcp 10.1.1.10 12345 198.51.100.10 80
  

These commands help validate translation entries and identify mismatches. `packet-tracer` simulates a packet path through the ASA and is valuable for pinpointing dropped or misrouted flows.

Conclusion

While NAT and PAT both translate addresses, their use cases in enterprise design differ. PAT is preferred for outbound scale, NAT for controlled inbound publishing. A strong understanding of these distinctions helps firewall administrators maintain secure, scalable, and predictable connectivity across edge networks.

Eduardo Wnorowski is a network infrastructure consultant and technologist.
With over 15 years of experience in IT and consulting, he brings deep expertise in networking, security, infrastructure, and transformation.
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