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Standard Access Control Lists (ACLs) Configuration

CCNA: 0 to Hero - Comprehensive Network Engineering Bootcamp

Lesson 49: Standard Access Control Lists (ACLs)

Access Control Lists (ACLs) are ordered lists of permit or deny statements (rules) applied to router interfaces to filter traffic based on predefined criteria.

Standard ACLs (Numbered 1-99 or 1300-1999)

  • Criterion: Can only filter based on the Source IPv4 Address.
  • Placement Rule: Place standard ACLs as close to the destination as possible to avoid accidentally blocking legitimate traffic.

The Implicit Deny Rule

Crucial Rule: Every ACL ends with an implicit deny any statement, meaning if a packet does not match any explicitly configured statement, it is automatically dropped.

Configuration Steps

  1. Create the ACL: ios ! ACL 10 denies host 192.168.1.5, but permits everything else Router(config)# access-list 10 deny host 192.168.1.5 Router(config)# access-list 10 permit any

  2. Apply to an Interface: Applied inbound or outbound. ios ! Apply inbound traffic filtering on the destination router interface Router(config)# interface G0/0 Router(config-if)# ip access-group 10 in