The answer is 0.0.0.0. In a standard IPv4 ACL, when no wildcard mask is explicitly configured, the router defaults to a wildcard mask of 0.0.0.0, which forces an exact host match against the source IP address. This occurs because the access-list command interprets the source address as a single host when no mask follows it, effectively applying the host wildcard mask that requires every bit of the address to match. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of ACL syntax and implicit behavior, often appearing in configuration scenarios where a missing mask leads to unintended host-only filtering. A common trap is assuming a default wildcard mask of 255.255.255.255 (any match) or 0.0.255.255 (subnet match); instead, remember that omitting the mask locks the rule to one specific host. Memory tip: think of the default wildcard mask as "zero tolerance"—0.0.0.0 means zero bits are ignored, so only the exact host passes.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 standard IPv4 ACL, what is the default wildcard mask if none is explicitly configured?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
0.0.0.0 (host match)
In a standard IPv4 ACL, if no wildcard mask is explicitly configured, the default wildcard mask is 0.0.0.0, which means the ACL matches only the exact source IP address (a host match). This is because the access-list command treats the source address as a host address when no mask is provided, effectively applying a host wildcard mask.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
0.0.0.0 (host match)
Why this is correct
When no wildcard mask is given, the router assumes a host match.
Related concept
Read the scenario before looking for a memorised answer.
✗
255.255.255.255 (any match)
Why it's wrong here
That would match all addresses, but it is not the default.
✗
The mask is mandatory; configuration is rejected without it.
Why it's wrong here
IOS accepts the command without mask and defaults to host match.
✗
The mask is derived from the source address class.
Why it's wrong here
No classful derivation occurs; it defaults to host.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a missing wildcard mask defaults to 'any' (255.255.255.255) or that the mask is mandatory, when in fact the default is a host match (0.0.0.0).
Trap categories for this question
Command / output trap
IOS accepts the command without mask and defaults to host match.
Detailed technical explanation
How to think about this question
Under the hood, when you configure 'access-list 1 permit 192.168.1.1', Cisco IOS internally appends a wildcard mask of 0.0.0.0, equivalent to 'host 192.168.1.1'. This behavior is consistent across standard and extended ACLs for the source address field. In real-world scenarios, forgetting to specify a mask can lead to unintended host-specific matches, which is critical when you intend to permit a subnet but omit the mask, causing only the single host to be permitted.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 0.0.0.0 (host match) — In a standard IPv4 ACL, if no wildcard mask is explicitly configured, the default wildcard mask is 0.0.0.0, which means the ACL matches only the exact source IP address (a host match). This is because the access-list command treats the source address as a host address when no mask is provided, effectively applying a host wildcard mask.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.