CCNA Network Infrastructure and Connectivity Practice Question
A host address is 2001:db8:100:20::25/64. Which portion identifies the network prefix?
⚠ Common exam trap
Be careful not to confuse the shorthand notation '::' with part of the network prefix. Remember, '::' represents a series of zeroes and is not part of the network prefix.
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
✓
2001:db8:100:20
With a /64 prefix, the first 64 bits identify the network. That corresponds to the first four hextets: 2001:db8:100:20.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
2001:db8:100
Why it's wrong here
This is only three hextets, or 48 bits, which would be a /48 prefix—a larger allocation block that contains many /64 subnets. It is not the /64 network portion, because it omits the fourth hextet (100:20) that is part of the subnet identifier. To specify the network, you must include all four hextets up to the /64 boundary.
When this WOULD be correct
In a question asking for the network prefix of an IPv6 address with a /48 subnet mask, such as 2001:db8:100::/48, option A would be correct as it represents the first three hextets that define the network portion under that specific subnetting scheme.
- ✓
2001:db8:100:20
Why this is correct
The first four hextets in the address represent the 64-bit network prefix, so 2001:db8:100:20 identifies the subnet. In IPv6, a /64 prefix always uses exactly four hextets, and the remaining four hextets (::25) are the interface identifier. This is the portion that routers use to forward traffic to the destination subnet.
- ✗
2001:db8:100:20::25
Why it's wrong here
This is the complete IPv6 address, combining the 64-bit network prefix (2001:db8:100:20) with the 64-bit interface identifier (::25). It is not a valid network portion, because a network prefix represents only the subnet, not the entire address. The network portion would be just the first four hextets without the suffix ::25.
When this WOULD be correct
In a different question asking for the full IPv6 address or the complete representation of the address, option C would be correct. For instance, if the question were 'What is the complete IPv6 address for the host?', then option C would accurately represent that.
- ✗
::25
Why it's wrong here
This is the uncompressed interface identifier, which is the last 64 bits of the address representing the host on the network. It does not include any of the network prefix, so selecting it as the network portion would be confusing the host part with the subnet. A network prefix must start at the beginning of the address and include the first 64 bits, not just the ending suffix.
When this WOULD be correct
If the exam question asked for the host identifier in the context of a subnetting question, where the focus was on identifying the host portion of an IPv6 address, then '::25' would be the correct answer, as it specifies the unique host within the given network.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓2001:db8:100:20Correct answer▾
Why this is correct
The first four hextets in the address represent the 64-bit network prefix, so 2001:db8:100:20 identifies the subnet. In IPv6, a /64 prefix always uses exactly four hextets, and the remaining four hextets (::25) are the interface identifier. This is the portion that routers use to forward traffic to the destination subnet.
✗2001:db8:100Wrong answer — click to see why▾
Why this is wrong here
Option A is incorrect because it only includes the first three hextets of the IPv6 address, which does not represent the complete network prefix as defined by the /64 subnet mask. The correct network prefix includes the first four hextets.
★ When this WOULD be the correct answer
In a question asking for the network prefix of an IPv6 address with a /48 subnet mask, such as 2001:db8:100::/48, option A would be correct as it represents the first three hextets that define the network portion under that specific subnetting scheme.
Why candidates choose this
Candidates may choose this option because they recognize that the first three hextets are part of the address structure, leading to confusion about what constitutes the full network prefix in different subnetting contexts.
✗2001:db8:100:20::25Wrong answer — click to see why▾
Why this is wrong here
Option C is incorrect because it includes the full address, which encompasses both the network prefix and the host portion. The question specifically asks for the network prefix, which is only the first 64 bits of the address.
★ When this WOULD be the correct answer
In a different question asking for the full IPv6 address or the complete representation of the address, option C would be correct. For instance, if the question were 'What is the complete IPv6 address for the host?', then option C would accurately represent that.
Why candidates choose this
Candidates may be tempted to choose this option because it appears to be the full address, leading them to mistakenly believe it includes the necessary network prefix information when it actually includes the host part as well.
✗::25Wrong answer — click to see why▾
Why this is wrong here
Option D, '::25', is incorrect because it represents the host portion of the IPv6 address, not the network prefix. The network prefix is determined by the first 64 bits, which in this case is '2001:db8:100:20'.
★ When this WOULD be the correct answer
If the exam question asked for the host identifier in the context of a subnetting question, where the focus was on identifying the host portion of an IPv6 address, then '::25' would be the correct answer, as it specifies the unique host within the given network.
Why candidates choose this
Candidates may be tempted to choose this option because they might confuse the shorthand notation for the host part with the overall address structure, leading them to mistakenly identify it as relevant to the network prefix.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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