This 300-410 practice question tests your understanding of ipv6 tunneling techniques. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
In a 6to4 tunnel, what is the default IPv6 prefix assigned to the tunnel interface?
2002::/16
Why wrong: 2002::/16 is the prefix for 6to4, but the tunnel interface uses a /48 derived from the source IPv4 address.
2002:IPv4-address::/48
The tunnel interface gets a /48 prefix based on its source IPv4 address.
2001::/32
Why wrong: 2001::/32 is used for Teredo, not 6to4.
3ffe::/16
Why wrong: 3ffe::/16 was used for 6bone, not 6to4.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
2002:IPv4-address::/48
In a 6to4 tunnel, the default IPv6 prefix assigned to the tunnel interface is 2002:IPv4-address::/48, where the IPv4 address of the tunnel source is embedded in the prefix. This is defined in RFC 3056, which specifies that the 6to4 prefix is 2002::/16, and the next 32 bits are the tunnel source's IPv4 address, resulting in a /48 prefix for the 6to4 site.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
2002::/16
Why it's wrong here
2002::/16 is the prefix for 6to4, but the tunnel interface uses a /48 derived from the source IPv4 address.
2002:IPv4-address::/48
Why this is correct
The tunnel interface gets a /48 prefix based on its source IPv4 address.
Related concept
Read the scenario before looking for a memorised answer.
2001::/32
Why it's wrong here
2001::/32 is used for Teredo, not 6to4.
3ffe::/16
Why it's wrong here
3ffe::/16 was used for 6bone, not 6to4.
Common exam traps
Cisco often tests the distinction between the 6to4 prefix range (2002::/16) and the actual prefix assigned to the tunnel interface (2002:IPv4-address::/48), leading candidates to mistakenly select the broader /16 prefix instead of the correct /48 derived from the IPv4 address.
Detailed technical explanation
The 6to4 tunnel interface automatically derives its IPv6 prefix by concatenating 2002::/16 with the 32-bit IPv4 address of the tunnel source, yielding a /48 prefix (e.g., 2002:ac10:0100::/48 for IPv4 172.16.1.0). This prefix is used for all IPv6 traffic sent over the tunnel, and the router must also have a static or dynamic route to the 2002::/16 network to reach other 6to4 sites. In real-world scenarios, 6to4 is often used for connecting isolated IPv6 islands over an IPv4-only backbone, but it requires public IPv4 addresses and may suffer from NAT traversal issues.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
The correct answer is: 2002:IPv4-address::/48 — In a 6to4 tunnel, the default IPv6 prefix assigned to the tunnel interface is 2002:IPv4-address::/48, where the IPv4 address of the tunnel source is embedded in the prefix. This is defined in RFC 3056, which specifies that the 6to4 prefix is 2002::/16, and the next 32 bits are the tunnel source's IPv4 address, resulting in a /48 prefix for the 6to4 site.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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