CCNA ACL Questions

The order of ACL entries is crucial; the first match is applied. Here, the permit any any matches all SMTP before the deny can block the specific traffic.

The deny any any matches all SMTP, so the subsequent permit for the specific source/destination is never reached.

The permit ip any any matches all traffic, so the subsequent deny for SMTP is never applied.

The ACL is applied outbound on G0/0 instead of inbound on G0/1. Although it may achieve the goal, it is not the most efficient placement and may not be the expected answer in a PBQ.

Missing the explicit deny statement. The implicit deny at the end of the ACL will block other traffic, but the question expects an explicit deny for clarity and completeness.

The deny statement is too broad; it denies all IP traffic, not just traffic to the loopback. The correct deny should be specific to the loopback host.

This option denies UDP traffic on port 23, but Telnet uses TCP, not UDP. Therefore, this ACL entry will not block Telnet sessions and is incorrect.

This option denies TCP traffic destined to the 10.20.30.0/24 subnet on port 23, which blocks inbound Telnet sessions to the subnet but does not prevent users inside the subnet from initiating outbound Telnet sessions.

This option permits TCP traffic from the subnet to any destination on port 80 (HTTP), allowing web browsing but does not block Telnet traffic, so it does not meet the requirement.

Standard ACLs can be applied near the destination or source, but the placement guidance for extended ACLs is based on their ability to filter on source and destination IP addresses and ports, not on limitations of standard ACLs. The reason for placing extended ACLs near the source is to filter traffic early, not because standard ACLs cannot be applied near the destination.

NAT translation is typically performed on routers or firewalls at network boundaries, and ACL placement for filtering is independent of NAT configuration. Placing an extended ACL near the source does not directly affect NAT translation; NAT uses its own rules and is not a factor in ACL placement decisions.

Extended ACLs can be applied inbound or outbound on any interface, not just inbound on access interfaces. The statement is factually incorrect; extended ACLs are versatile and can be placed in various locations depending on the filtering requirements.

The specific factual error is that standard ACLs cannot filter by protocol (ICMP). They only match source IP addresses.

The specific factual error is that the ACL statement has the source and destination swapped, and it is applied on the wrong interface (G0/0 instead of G0/1).

The specific factual error is that the existing ACL 10 is too permissive and is applied on the wrong interface (G0/0 inbound) to filter traffic from R3 to R2. It must be removed to enforce the new restrictions.

The ACL is applied on the wrong interface; it should be applied inbound on G0/0, not G0/1.

The ACL is applied in the wrong direction; it should be inbound, not outbound.

The ACL is applied on an extra interface (G0/1) that is not required, which may block legitimate traffic.

Standard ACLs are specifically designed to match source IP addresses, so this statement is factually incorrect. They can match source addresses using wildcard masks.

Named ACLs use wildcard masks just like numbered ACLs; the naming does not affect the functionality of wildcard masks. The statement is technically incorrect.

The 'deny any' line at the end of an ACL is implicit, so placing it before the permit would block all traffic, including the intended 192.168.30.0/24 subnet. The order of entries matters, but the issue here is not about the order of deny any.

The specific factual error is that the existing ACL contains a permit ip any any statement that would override any specific denies, allowing all traffic.

The specific factual error is that ACLs should be applied inbound on the interface where the traffic enters the router, not on the outbound interface towards the destination.

The specific factual error is that outbound ACLs filter traffic exiting the interface, but the traffic from LAN to server enters the router via GigabitEthernet0/0 and exits via another interface (e.g., WAN). An outbound ACL on GigabitEthernet0/0 would not affect traffic going to the server.

The wildcard mask 255.255.255.0 would match only the exact host 192.168.10.0, not the entire /24 subnet. Cisco ACLs use wildcard masks where 0 means match and 1 means ignore; for a /24, the correct mask is 0.0.0.255.

Standard ACLs can only filter based on source IP address and cannot match specific protocols like Telnet (TCP port 23) or destination addresses. Extended ACLs are required to filter Telnet traffic from a specific source to a specific destination.

PAT (Port Address Translation) is unrelated to ACL filtering. ACLs operate independently of NAT/PAT; they filter traffic based on Layer 3 and Layer 4 information regardless of whether translation is configured.

Standard ACLs filter only on source IP, so placing them close to the source can block traffic destined to other networks that should be allowed. The correct placement is close to the destination to minimize unintended filtering.

Standard ACLs do not examine destination IP addresses; they only match on source IP addresses. Filtering by destination requires an extended ACL.

Standard ACLs operate at Layer 3 and cannot examine Layer 4 information such as TCP or UDP port numbers. Port-based filtering requires an extended ACL.

Standard ACLs filter only on source IP address, so placing them close to the source can block traffic that should be allowed to other destinations, causing unnecessary denial of service.

A wildcard mask of 255.255.255.255 means 'ignore all bits,' so it matches any IP address, equivalent to the 'any' keyword. It does not match all bits.

Extended ACLs are more effective when placed near the source to filter unwanted traffic early, not near the destination. Placing them near the destination allows unwanted traffic to traverse the network unnecessarily.

Standard ACLs can only filter based on source IP address, not destination ports or protocols. They lack the granularity to distinguish between Telnet and SSH traffic.

Extended ACLs are not required for every router login policy; they are only needed when filtering must consider protocol or port information. Simple source-based filtering can use standard ACLs.

SSH uses TCP port 22, while Telnet uses TCP port 23. They are distinct ports, so an ACL can differentiate them based on destination port.

The deny ip any any statement overrides the permit statements because ACLs are processed sequentially; any packet matching the deny is dropped. Also, applying the ACL inbound on the server-facing interface would filter traffic coming from the servers, not from the 192.168.1.0/24 subnet.

The ACL should be applied inbound on the interface receiving traffic from the 192.168.1.0/24 subnet (GigabitEthernet0/0) to filter before routing. Applying it outbound on the same interface would only filter traffic leaving that interface, which is not the traffic path from the subnet to the servers.

The ACL must be applied on the interface closest to the source of the traffic to be filtered. Since the traffic originates from the 192.168.1.0/24 subnet, the ACL should be applied inbound on GigabitEthernet0/0, not on the server-facing interface.

Candidates may assume any ACL misbehavior is due to wrong interface direction, ignoring that the specific symptom (only 192.168.20.0 is unreachable) points to the deny rule itself.

Candidates often focus on what an ACL ‘lacks’ rather than the sequence, missing that the existing permit any covers the destination but is shadowed by the earlier deny.

Candidates might recall that all ACLs have an implicit deny, but they fail to realize that a packet matching an earlier deny is already discarded, and the implicit deny only applies to unmatched traffic.

The ACL is applied in the wrong direction. Applying it outbound would filter traffic leaving the interface, not entering it.

The ACL permits all ICMP traffic instead of only echo-reply. This violates the requirement to permit only ICMP echo-reply.

Applying the ACL to an additional interface (G0/1) is unnecessary and may cause unintended filtering. The requirement specifies only one interface.

The specific factual error is that the permit statement does not exclude ports 80 and 443. It allows all TCP ports, which is too permissive.

The specific factual error is that the permit statements allow the very ports that should be denied for VLAN 20. This misinterprets the requirement.

The specific factual error is that the permit statement is too broad and overrides the deny statements. The correct approach is to permit only non-HTTP/HTTPS ports, not all ports.

Misunderstanding of ACL direction leads candidates to think that the implicit deny blocks any traffic leaving the interface.

Confusing ACL filtering with routing functionality; ACLs do not prevent the router from routing between connected subnets unless they explicitly deny the traffic on the appropriate interface and direction.

Candidates often try to add a permit for the source subnet of the initiating traffic, neglecting the direction of the ACL. Because the ACL is inbound on the egress interface, outbound traffic is not filtered.

Applying the ACL outbound would not change the order of lines; the same logic applies—the first match still blocks all traffic.

Extended ACLs can match TCP port 23 using the keyword 'eq telnet' or 'eq 23'; this is not a limitation.

Telnet uses TCP, not UDP; referencing udp would never match Telnet traffic.

This range uses a wildcard mask of 0.0.0.7, not 0.0.0.15.

This option is incorrect because the ISP-facing interface is correctly configured as ip nat outside. The problem is not with the interface NAT designation but with the ACL mismatch. Changing the inside interface designation would not fix the translation issue.

This option is incorrect because NAT overload commonly uses the IP address of the outside interface as the translated source address. This is standard Cisco NAT behavior and not a cause of failure in this scenario.

This option is incorrect because NAT does not depend on routing protocols like OSPF to function. NAT translation is independent of routing protocols, so running OSPF is not required for NAT overload to work.

Candidates may assume the ACL is not applied correctly, but without interface details this conclusion cannot be drawn from the given output.

Candidates might blame the server configuration rather than the network ACL, but the question stem provides the server state to rule this out.

This is a common misconception: the explicit deny is not the root cause; the missing permit is the real issue. Removing the deny without adding a permit for HTTPS would still result in the traffic being blocked by the implicit deny.

Leaving the standard ACL inbound on the LAN interface is incorrect because standard ACLs filter only by source IP, which causes all traffic from that host, including local LAN traffic, to be blocked. This disrupts local communications and is not best practice.

Converting the ACL to a VTY access-class is irrelevant to the question because VTY access-classes control remote management access to the router, not general traffic filtering from a host to remote networks.

Applying the ACL inbound on all switch access ports is impractical and inefficient. It would block traffic at multiple points unnecessarily and does not address the specific need to filter traffic from the host to remote networks.

The direction of ACL application is correct; inbound on the interface where traffic enters the router is standard for filtering traffic from the LAN.

ACLs should be applied as close to the source as possible to deny traffic early. Applying on the serial interface would still work but is less efficient and not the best practice.

The permit ip any any allows all traffic, so HTTPS should be allowed. The problem says HTTPS is blocked, so the issue must be something else, like a missing explicit permit or a misordering.

Option C applies the new ACL without explicitly removing the old one, which violates the explicit removal requirement.

Candidates assume that any missing traffic must be explicitly permitted, but this ignores the need for stateful inspection and leads to an overly permissive rule.

Candidates mistakenly think that moving the ACL to inbound direction will inherently permit return traffic because it inspects packets entering the interface, but the ACL still processes each packet individually without keeping state.

Candidates may think that only a stateful firewall can handle return traffic, overlooking the established keyword's capability to emulate stateful behavior for TCP traffic on an ACL.

This describes the capability of extended ACLs, not standard ACLs.

Protocol filtering is a feature of extended ACLs, not standard ACLs.

Both inbound and outbound application are possible, though placement depends on the filtering strategy.

Candidates may confuse security inspection with NAT translation, assuming that a protocol‑specific inspection is needed for HTTP.

Tempting because an ACL could selectively block TCP; however, the question provides context about the renumbering, which directly points to the NAT configuration.

Candidates may assume that different protocols might take different paths, but a single default route applies uniformly to all IP traffic.

This option is wrong because the ACL only permits TCP traffic on port 443 and ICMP traffic, so an HTTP packet (port 80) from the specified source would be denied by the implicit deny rule at the end of the ACL.

This option is wrong because NAT does not occur before ACL evaluation; the ACL is applied directly to the packet as it arrives at the interface. Therefore, the HTTP packet is evaluated against the ACL without any translation taking place.

This option is wrong because the ACL does not automatically convert HTTP traffic to HTTPS; it only permits or denies traffic based on the defined rules. The packet from 10.10.10.25 is not permitted by the ACL since it is not targeting port 443.

A common mistake is to try to explain zero matches by directional misapplication without first checking whether an ACL is actually present. The exhibit explicitly shows no ACL is bound.

Some candidates fixate on ACL configuration details instead of verifying interface assignment. The output confirms the interface has no ACL, not that an ACL is configured incorrectly.

Candidates sometimes misread interface status. This output clearly shows the interface is enabled and up, so a down state is not the issue.

Incorrect. Standard ACLs cannot match TCP or UDP port numbers; this capability belongs to extended ACLs, which provide more granular filtering.

Incorrect. ACLs do not automatically create a 'permit any' at the end; instead, they have an implicit deny all statement that blocks any traffic not explicitly permitted.

Applying 'ip nat enable' on both interfaces is incorrect because Cisco IOS uses 'ip nat inside' and 'ip nat outside' to define NAT roles. The problem is not enabling NAT but assigning the correct directional roles to interfaces.

Replacing 'overload' with a pool is unnecessary since PAT uses 'overload' to allow multiple inside hosts to share one outside IP. The issue is not the translation method but the reversed inside/outside interface roles.

Changing access-list 1 to a standard ACL numbered 100 does not address the core problem. ACL 1 is valid for identifying inside local addresses, and the failure is due to reversed NAT interface roles, not the ACL number.

Candidates may think that the ACL itself, if still configured, continues to filter traffic even when not applied to an interface.

A common misunderstanding is that the access-group line can persist without a valid ACL and cause some default behavior; in fact the entire command is removed.

Some candidates may confuse direction changes with removal and assume the ACL is still filtering traffic in some way, but the symptom clearly indicates no filtering at all.

Matching only one source subnet can be accomplished with a standard ACL, so this does not strongly indicate a need for an extended ACL.

Placing an ACL near the destination is a characteristic of standard ACLs, not a criterion that selects an extended ACL.

The scenario assumes both standard and extended ACLs are available; standard ACLs do not exist for IPv6, so this requirement does not apply to the IPv4 ACL choice.

Standard ACLs are not limited to wireless networks; they can be applied to any interface on a router, including wired connections. This option misrepresents the applicability of standard ACLs.

This option is incorrect because Telnet and SSH use different destination ports; Telnet typically uses port 23, while SSH uses port 22, allowing for distinct filtering in ACLs.

This option is incorrect because SSH can indeed be filtered using ACLs, as they can match traffic based on IP addresses and protocols. Standard ACLs can be applied to control SSH traffic just like any other traffic type.

The scenario explicitly mentions successful pings to IP addresses, so name resolution is not the cause.

The command output explicitly shows 'overload', and the symptom (ping works, TCP fails) is inconsistent with a missing overload keyword.

A static NAT conflict would affect all traffic (including ICMP), not just web traffic.

Placing an extended ACL as close to the destination can lead to unnecessary traffic being processed by intermediate devices, which is inefficient. Extended ACLs are designed to filter traffic based on source and destination, so positioning them closer to the source enhances performance and security.

This option is incorrect because placing an extended ACL only on the default gateway limits its effectiveness in controlling traffic originating from various sources across the network. Extended ACLs should be strategically placed closer to the source to filter traffic before it reaches the destination.

Placing an extended ACL only on WAN interfaces can lead to inefficient traffic filtering, as it may not adequately control traffic originating from internal sources. Extended ACLs should ideally be positioned close to the source to effectively manage traffic before it reaches the destination.