The answer is that standard ACLs are best placed near the source in most cases, not near the destination. This is because a standard ACL only examines the source IP address of a packet, lacking the ability to filter based on destination or port. When applied close to the destination, as described in the scenario, it will block all traffic from that specific host to any destination beyond the ACL, rather than just the intended target, which is why the design is flawed. On the CCNA 200-301 v2 exam, this concept tests your understanding of ACL placement logic and the inherent limitation of standard numbered ACLs (1-99 and 1300-1999). A common trap is assuming you can fine-tune standard ACLs like extended ACLs, but the key is remembering they are source-only filters. Memory tip: Standard ACLs are like a bouncer who only checks your ID (source IP) at the door—if you place that bouncer at the back of the club (near destination), he stops everyone you came with, not just you.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. A key principle to apply: standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.. 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 standard numbered ACL is applied close to the destination, but it is blocking traffic from one host while still allowing all other users on the subnet. Which two facts about standard ACLs are relevant in this design?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
They filter based on source address only
Standard ACLs only match the source IP address. For that reason, they are usually placed near the source so they do not block more traffic than intended.
Key principle: Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
They filter based on source address only
Why this is correct
Standard ACLs do not inspect destination addresses or ports.
Related concept
Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
✓
They are best placed near the source in most cases
Why this is correct
Because they only match source IPs, placing them near the source reduces unwanted broad blocking.
Related concept
Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
✗
They can match TCP and UDP port numbers
Why it's wrong here
That is the role of extended ACLs.
✗
They automatically create a permit any at the end
Why it's wrong here
ACLs have an implicit deny at the end.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that standard ACLs can filter traffic based on destination IP addresses or port numbers. Candidates may incorrectly place standard ACLs near the destination to control traffic more granularly, but since standard ACLs only match source IPs, this placement can block unintended hosts. This misunderstanding leads to unexpected network outages or partial connectivity, especially when trying to block a single host but inadvertently affecting others in the subnet. Recognizing the source-only filtering nature of standard ACLs is essential to avoid this pitfall.
Detailed technical explanation
How to think about this question
Standard Access Control Lists (ACLs) in Cisco networking filter traffic based solely on the source IP address of packets. Unlike extended ACLs, they do not consider destination IP addresses, protocols, or port numbers. This fundamental design means that standard ACLs apply broad filtering rules that affect all traffic originating from specified source addresses, regardless of where it is headed or what service it uses.
Because standard ACLs only match on source IP addresses, best practice dictates placing them as close to the source of traffic as possible. This placement prevents unintended blocking of legitimate traffic from other hosts within the same subnet or network segment. Applying a standard ACL near the destination can inadvertently block traffic from multiple sources, since the ACL cannot differentiate based on destination or service, leading to overly restrictive filtering.
A common exam trap involves misplacing standard ACLs near the destination, expecting granular control over traffic. This mistake ignores the ACL’s limitation to source-based filtering and can cause unexpected blocking of hosts. Understanding this behaviour is critical for designing effective ACLs in Cisco environments and avoiding network connectivity issues during CCNA-level configurations.
KKey Concepts to Remember
Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
Because standard ACLs only inspect source IPs, placing them near the source prevents unintended blocking of other hosts in the subnet.
Applying a standard ACL near the destination risks blocking more traffic than intended due to its limited filtering criteria.
Extended ACLs are required to filter traffic based on destination IP addresses, protocols, or TCP/UDP port numbers.
All ACLs have an implicit deny at the end, meaning any traffic not explicitly permitted is blocked.
Standard ACLs do not inspect or filter based on TCP or UDP port numbers, limiting their granularity.
Proper ACL placement is critical to avoid network disruptions and ensure intended traffic flow in Cisco routing environments.
Misunderstanding standard ACL behaviour can lead to configuration errors and connectivity issues in Cisco networks.
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
Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports..
What is the correct answer to this question?
The correct answer is: They filter based on source address only — Standard ACLs only match the source IP address. For that reason, they are usually placed near the source so they do not block more traffic than intended.
What should I do if I get this 200-301 question wrong?
Review standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Standard ACLs filter traffic exclusively based on the source IP address of packets, ignoring destination addresses and ports.
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