CAS-004 Security Engineering Practice Question
A network administrator needs to establish a secure VPN tunnel between two branch offices using IPsec. The requirement is to encrypt the entire IP packet, including the original IP header. Which IPsec mode should be used?
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
✓
Tunnel mode
Tunnel mode encrypts the entire IP packet and adds a new IP header, making it suitable for VPNs between networks.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
AH mode
Why it's wrong here
AH provides integrity but not encryption.
- ✗
IKEv1
Why it's wrong here
IKEv1 is a key-exchange protocol that negotiates security associations; it defines no packet-encapsulation mode, so it cannot encrypt the original IP header. It is tempting because IKEv1 commonly accompanies IPsec VPNs, but it handles authentication and key management, not the encapsulation itself.
- ✓
Tunnel mode
Why this is correct
Tunnel mode encapsulates the complete original IP packet inside a new IP packet, encrypting payload and original header together. Transport mode encrypts only the payload and leaves the original header intact, so tunnel mode is required to meet the stated requirement of encrypting the entire packet.
- ✗
Transport mode
Why it's wrong here
Transport mode encrypts only the payload and leaves the original IP header intact and unencrypted; tunnel mode encapsulates and encrypts the entire original packet. It is tempting because transport mode suits host-to-host IPsec, but it fails the branch-to-branch requirement to conceal the original header.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CAS-005 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CAS-005 exam.