This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: iPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.. 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.
A host address is 2001:db8:100:20::25/64. Which portion identifies the network prefix?
2001:db8:100
Why wrong: Three hextets represent only 48 bits, not 64.
2001:db8:100:20
Correct. Four hextets make up the /64 network prefix here.
2001:db8:100:20::25
Why wrong: This is the full address, not just the network portion.
::25
Why wrong: The last interface identifier portion does not define the network prefix.
Answer choices
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.
Key principle: IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
Answer analysis
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
Three hextets represent only 48 bits, not 64.
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
Correct. Four hextets make up the /64 network prefix here.
Related concept
IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
2001:db8:100:20::25
Why it's wrong here
This is the full address, not just the network portion.
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
The last interface identifier portion does not define the network prefix.
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
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.
Why this is correct
Correct. Four hextets make up the /64 network prefix here.
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.
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.
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?”
Common exam traps
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.
Detailed technical explanation
IPv6 addressing uses 128-bit addresses divided into eight 16-bit blocks called hextets. The network prefix portion of an IPv6 address defines the network segment and is specified by the prefix length, such as /64, which means the first 64 bits (or first four hextets) represent the network. The remaining bits identify the interface or host within that network. This structure allows for hierarchical routing and efficient address aggregation in IPv6 networks, which is fundamental knowledge for the CCNA exam. In this question, the address 2001:db8:100:20::25/64 uses a /64 prefix length, meaning the first 64 bits—equivalent to the first four hextets (2001:db8:100:20)—define the network prefix. The remaining bits after the fourth hextet represent the interface identifier. Cisco devices and IPv6 standards typically use /64 prefixes for LAN segments, making this a standard subnetting practice in IPv6 addressing. A common exam trap is confusing the network prefix with the full address or only part of it. For example, selecting only three hextets (48 bits) or including the interface identifier leads to incorrect answers. Understanding that the /64 prefix always covers the first four hextets helps avoid this mistake. Practically, Cisco routers use this prefix length for routing decisions and subnetting, so recognizing the network portion is critical for configuring IPv6 interfaces and troubleshooting connectivity.
Key takeaway
IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
Real-world example
A practitioner preparing for the 200-301 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. IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Review iPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions..
The correct answer is: 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.
Review iPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions., then practise related 200-301 questions on the same topic to reinforce the concept.
IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
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Last reviewed: May 17, 2026
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