mediumMultiple Choice
6to4 Automatic Tunnel: Addressing and Loop Prevention
In a 6to4 tunnel, how is the tunnel destination address determined?
⚠ Common exam trap
Cisco often tests the distinction between automatic 6to4 tunnels and manually configured tunnels, where candidates mistakenly think all tunnels require manual destination configuration, but 6to4 uniquely derives it from the IPv6 address.
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
✓
It is derived from the IPv6 destination address using the 2002::/16 prefix.
In a 6to4 tunnel, the tunnel destination address is automatically derived from the IPv6 destination address by extracting the IPv4 address embedded in the 2002::/16 prefix. Specifically, the first 16 bits of the IPv6 destination are 2002, and the next 32 bits represent the IPv4 address of the remote 6to4 router. This allows the tunnel to be dynamically established without manual configuration of each destination.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It is manually configured.
Why it's wrong here
In 6to4, the tunnel destination is derived automatically by extracting the IPv4 address embedded in the 2002::/16 destination address, not configured by hand. Manual configuration is tempting because it applies to explicit IPv6-in-IPv4 tunnels such as GRE or manual 6in4, where endpoints are statically defined.
- ✓
It is derived from the IPv6 destination address using the 2002::/16 prefix.
Why this is correct
6to4 embeds the IPv4 tunnel endpoint inside the IPv6 address itself: the 2002::/16 prefix is followed by the 32-bit IPv4 address. The router extracts those bits from the packet's IPv6 destination to determine the tunnel endpoint, requiring no explicit destination configuration.
- ✗
It is obtained via DNS.
Why it's wrong here
DNS is not used for 6to4 destination.
- ✗
It is the same as the tunnel source.
Why it's wrong here
The destination is the remote 6to4 border router's IPv4 address embedded in the 2002::/16 prefix, not the local source. Using the source would loop traffic back; source equals destination only in loopback testing, never in production tunnelling.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.