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Difficulty: EasyAccess Control Lists (Standard and Extended IPv4 ACLs)

A network administrator applies the following standard IPv4 access control list (ACL) inbound on a router interface:

`access-list 10 permit 192.168.1.0 0.0.0.255`

A host with the IPv4 address `192.168.2.50` attempts to send traffic through this interface. What action will the router take on this packet?

  1. The router drops the packet because it matches the implicit deny statement at the end of the ACL.Answer
  2. B
    The router permits the packet because no explicit deny statement was configured in the ACL.
  3. C
    The router forwards the packet to the default gateway for further inspection.
  4. D
    The router prompts the sending host for authentication before making a forwarding decision.

Answer

The router drops the packet because it matches the implicit deny statement at the end of the ACL.
Cisco IOS IPv4 Access Control Lists evaluate rules sequentially from top to bottom. If a packet does not match any configured rule, it reaches the final unwritten entry: the implicit deny (deny ip any). Because the source IP 192.168.2.50 is not in the 192.168.1.0/24 subnet specified in the single permit line, it is silently dropped by this implicit deny feature.

Step-by-Step Solution

1
Evaluate the packet source IPv4 address against the ACL entries.
The packet source address 192.168.2.50 does not match the permit rule for network 192.168.1.0/24.
Standard ACL 10 only matches host IP addresses falling within the 192.168.1.0 to 192.168.1.255 range.
2
Apply default ACL processing logic when no explicit entries match.
The packet reaches the bottom of the ACL and hits the implicit deny clause.
All Cisco IPv4 ACLs end with an unwritten 'deny ip any' rule that drops any traffic not explicitly permitted.

Key Concept

Access Control List (ACL) Implicit Deny Clause
Estimated Time:45s
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Topics

Access Control Lists (Standard and Extended IPv4 ACLs)

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