Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
A host address is 2001:db8:100:20::25/64. Which portion identifies the network prefix?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
2001:db8:100
Three hextets represent only 48 bits, not 64.
B
Best answer
2001:db8:100:20
Correct. Four hextets make up the /64 network prefix here.
C
Distractor review
2001:db8:100:20::25
This is the full address, not just the network portion.
D
Distractor review
::25
The last interface identifier portion does not define the network prefix.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting only the first three hextets (48 bits) as the network prefix instead of the correct four hextets (64 bits). This mistake arises because candidates sometimes assume the network portion corresponds to three hextets, similar to IPv4's classful addressing, but IPv6 uses a fixed /64 prefix for most subnets. Another trap is confusing the full IPv6 address, including the interface identifier (::25), as the network prefix. This leads to incorrect subnetting and routing assumptions. Recognizing that the /64 prefix always defines the first four hextets as the network portion is essential to avoid these pitfalls.
Technical deep dive
How to think about this question
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.
KKey Concepts to Remember
- IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
- The prefix length in IPv6, such as /64, specifies how many bits from the start of the address represent the network portion.
- A /64 prefix length corresponds to the first four hextets (64 bits) of an IPv6 address, defining the network segment.
- The remaining bits after the network prefix in an IPv6 address identify the host or interface within that network.
- Cisco devices typically use /64 prefixes for LAN subnets, making this prefix length a standard in IPv6 subnetting and routing.
- Confusing the full IPv6 address or fewer than four hextets as the network prefix leads to subnetting errors and routing misconfigurations.
- Understanding the network prefix length is critical for configuring IPv6 interfaces and troubleshooting IPv6 connectivity in Cisco networks.
- The network prefix portion is used by routers to make forwarding decisions and aggregate routes efficiently in IPv6 environments.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
IPv6 addresses consist of eight 16-bit hextets, totaling 128 bits, which are divided into network prefix and interface identifier portions.
What is the correct answer to this question?
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.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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