How Is A Source IP Address Used In A Standard ACL?

Introduction to Access Control Lists

Access Control Lists (ACLs) are a fundamental component of network security, used to filter traffic based on predefined rules. In Cisco networking, ACLs play a crucial role in permitting or denying traffic flow. A Standard ACL is one of the simplest types of ACLs, primarily filtering traffic based on the source IP address.

This article explores how a source IP address is used in a Standard ACL, its configuration, and its significance in network security. Additionally, we will discuss how Study4Pass provides high-quality Cisco 200-301 study material and CCNA CyberOps Associate (Version 1.0) resources to help aspiring network professionals master these concepts.

Understanding Standard ACLs

A Standard Access Control List (ACL) is a sequential list of permit or deny statements that control traffic based on:

• Source IP address
• Wildcard mask (optional, for grouping IP addresses)

Unlike Extended ACLs, which can filter based on source/destination IP, port numbers, and protocols, Standard ACLs only consider the source IP address.

Key Characteristics of Standard ACLs

1. Filters Traffic Based on Source IP Only – Does not consider destination IP, port, or protocol.
2. Uses Numbered (1-99, 1300-1999) or Named ACLs – Older Cisco devices use numbered ranges.
3. Processed Top-Down – The first matching rule is applied; if no match, an implicit deny any blocks all remaining traffic.
4. Best Placed Near the Destination – Since it filters based on source, placing it too close to the source may unintentionally block legitimate traffic.

How Source IP Address is Used in Standard ACLs?

The source IP address is the primary criterion in a Standard ACL. Below is a breakdown of its role:

1. Identifying Traffic Origin

• A Standard ACL checks the source IP field in the packet header.
• If the source IP matches an ACL rule, the corresponding permit/deny action is applied.

2. Wildcard Mask for IP Range Filtering

• A wildcard mask (inverse of subnet mask) specifies a range of IP addresses.
• Example:
• ACL Rule: access-list 10 permit 192.168.1.0 0.0.0.255
• Interpretation: Permits all traffic from 192.168.1.0/24 network.

3. Implicit Deny Any

• If no rule matches, Cisco applies an automatic deny any at the end.

• Example:

access-list 20 permit 10.1.1.1 

access-list 20 deny 10.1.1.2 

• Only 10.1.1.1 is allowed; all other IPs are blocked.

Configuring a Standard ACL in Cisco Devices

Here’s a step-by-step configuration example:

Step 1: Create the ACL

Router(config)# access-list 10 permit 192.168.1.1 

Router(config)# access-list 10 deny 192.168.1.2 

Router(config)# access-list 10 permit 192.168.1.0 0.0.0.255 

Step 2: Apply the ACL to an Interface

Router(config)# interface GigabitEthernet0/0 

Router(config-if)# ip access-group 10 in 

Step 3: Verify the ACL

Router# show access-lists 

Use Cases of Standard ACLs

1. Basic Traffic Filtering – Allow/block specific hosts.
2. Routing Updates Control – Restrict OSPF or EIGRP neighbors.
3. Security Policies – Prevent unauthorized access from certain subnets.

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Final Verdicts

A Standard ACL filters traffic based solely on the source IP address, making it a simple yet powerful tool for network security. Understanding its configuration and application is essential for CCNA 200-301 and CyberOps Associate exams.

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Actual exam question from Cisco's 200-301 Exam Questions.

Sample Questions for Cisco 200-301 CCNA certification

1. In a Standard ACL, what does the source IP address identify?

A) The destination of the traffic

B) The origin of the traffic

C) The protocol being used

D) The port number of the traffic

2. What is the primary function of a Standard ACL when using a source IP address?

A) Filter traffic based on destination IP

B) Filter traffic based on source IP only

C) Filter traffic based on both source and destination ports

D) Encrypt traffic from a specific source

3. Where should a Standard ACL (using source IP) typically be placed for optimal performance?

A) As close to the destination as possible

B) As close to the source as possible

C) On a core router only

D) On a switch instead of a router

4. Which of the following commands correctly applies a Standard ACL to block traffic from source IP 192.168.1.10?

A) access-list 10 deny 192.168.1.10 0.0.0.0

B) access-list 10 permit 192.168.1.10 255.255.255.255

C) access-list 100 deny tcp any any

D) access-list 10 deny host 192.168.1.10

5. What is a limitation of using a Standard ACL that filters only by source IP?

A) It cannot distinguish between different types of traffic (e.g., HTTP vs. FTP).

B) It encrypts the traffic from the source.

C) It filters based on destination ports.

D) It requires both source and destination IPs to function.