156-315.81.20 Advanced VPN Design Practice Question
An administrator needs to allow VPN traffic to pass through a NAT device. Which feature must be enabled in the VPN community settings?
⚠ Common exam trap
Candidates often confuse NAT-T with standard NAT rules. They fail to understand that NAT-T is a specific VPN encapsulation method required to prevent ESP packet drops.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
NAT-Traversal (NAT-T).
NAT-Traversal (NAT-T) is required when the VPN traffic passes through a device performing address translation, as the original IP headers are modified. NAT-T encapsulates the ESP packets within UDP port 4500, allowing the traffic to traverse the NAT device successfully without breaking the integrity of the IPsec connection. This is a standard requirement for remote access and site-to-site VPNs in environments where global IPs are limited.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
VPN Domain NAT.
Why it's wrong here
VPN Domain NAT is used to hide internal IP addresses behind a gateway's external interface. It does not provide the protocol-level encapsulation required to pass ESP packets through an intervening NAT device. It solves a different problem, specifically related to address overlaps, not NAT-Traversal for the tunnel itself.
- ✓
NAT-Traversal (NAT-T).
Why this is correct
NAT-Traversal (NAT-T) is the specific protocol mechanism that allows IPsec VPN tunnels to work when NAT is present. By wrapping the encrypted traffic in UDP headers, NAT-T enables the packets to bypass NAT devices that would otherwise drop them because the ESP protocol does not have ports for translation.
- ✗
IKEv1 Aggressive Mode.
Why it's wrong here
Aggressive mode is an IKEv1 handshake method that is generally considered less secure than Main Mode. It does not handle NAT-Traversal requirements. While it might sometimes work in specific NAT scenarios, it is not the correct feature to enable for NAT-Traversal, and it is discouraged due to security risks.
- ✗
Hide NAT on the VPN Gateway.
Why it's wrong here
Hide NAT is a static configuration for address translation and does not automatically handle the dynamic needs of NAT-Traversal for VPN tunnel traffic. Enabling Hide NAT on the gateway will not provide the encapsulation required for ESP traffic to pass through an external NAT device in the network path.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Check Point exam blueprint
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