What Wild card Mask Will Match Networks 172.16.0.0 Through 172.19.0.0?

Introduction

In the world of networking, understanding IP addressing and subnetting is crucial for both beginners and experienced professionals. One of the more specific yet highly useful aspects of subnetting is the concept of wildcard masks. These are particularly useful in Access Control Lists (ACLs), routing protocols like OSPF, and various network configurations where specifying a range of IP addresses is necessary. In this blog post, brought to you by DumpsArena, we will explore a particular networking question: "What wildcard mask will match networks 172.16.0.0 through 172.19.0.0?" This question may appear simple at first glance, but it requires a deep understanding of how IP ranges and wildcard masks work. Let us delve into the concept in detail and unravel the answer step by step.

What is a Wildcard Mask?

A wildcard mask is a mask of bits that indicates which parts of an IP address should be matched exactly and which parts can vary. Unlike a subnet mask, which is used to divide an IP address into network and host portions, a wildcard mask is used primarily for matching purposes in networking. It is widely used in Cisco configurations, especially in ACLs.

For example, if we want to permit or deny traffic from a specific range of IP addresses, we use a wildcard mask to specify that range efficiently. Wildcard masks work in a binary fashion, where:

A 0 bit means that the corresponding bit in the IP address must match exactly.
A 1 bit means that the corresponding bit can be anything (it is a wildcard).

Understanding the IP Address Range: 172.16.0.0 through 172.19.0.0

To determine the correct wildcard mask, we first need to understand what the IP address range 172.16.0.0 through 172.19.0.0 represents.

These addresses belong to the Class B private IP address range, specifically:

172.16.0.0 = 10101100.00010000.00000000.00000000
172.17.0.0 = 10101100.00010001.00000000.00000000
172.18.0.0 = 10101100.00010010.00000000.00000000
172.19.0.0 = 10101100.00010011.00000000.00000000

As we can see, the first octet (172) remains constant. The second octet changes from 16 to 19. Therefore, we need to create a wildcard mask that allows variation from 16 to 19 in the second octet.

Calculating the Wildcard Mask

To calculate the wildcard mask that matches the IP range from 172.16.0.0 to 172.19.0.0, follow these steps:

Determine the range in binary.
- 172.16.0.0 = 10101100.00010000.00000000.00000000
- 172.19.0.0 = 10101100.00010011.00000000.00000000
Compare the bits to identify where the changes occur.
- From the binary, it's evident that the first 20 bits remain constant and only bits 21 and 22 vary.
To cover a range of 4 values in the second octet (16 to 19), you can use a wildcard that allows for variation in those bits.
Thus, the subnet that can encapsulate this range is 172.16.0.0/14. This subnet includes:
- 172.16.0.0
- 172.17.0.0
- 172.18.0.0
- 172.19.0.0
The corresponding wildcard mask for /14 is 0.3.255.255:
- In binary: 00000000.00000011.11111111.11111111

So, the final answer to the question "What wildcard mask will match networks 172.16.0.0 through 172.19.0.0?" is 0.3.255.255.

Practical Example in a Cisco ACL

Imagine you are configuring an access list on a Cisco router and you want to permit traffic from networks 172.16.0.0 through 172.19.0.0. You can write the following line in your configuration:

access-list 10 permit 172.16.0.0 0.3.255.255

This single line effectively permits traffic from all four networks because the wildcard mask allows variation in the second octet from 16 through 19 and all values in the third and fourth octets.

Why Not Use Multiple Entries?

One might ask why not just write four separate lines for each network. While that's certainly possible, it's not efficient:

access-list 10 permit 172.16.0.0 0.0.255.255
access-list 10 permit 172.17.0.0 0.0.255.255
access-list 10 permit 172.18.0.0 0.0.255.255
access-list 10 permit 172.19.0.0 0.0.255.255

While these entries work, they consume more space and processing power. A single line using a well-calculated wildcard mask is more efficient and professional, which is critical in large enterprise networks. DumpsArena always recommends using optimized configurations whenever possible.

Additional Insights from DumpsArena

At DumpsArena, we help networking professionals understand the intricacies of IP addressing, subnetting, and wildcard masks. These are not just theoretical concepts but real-world skills necessary for configuring and troubleshooting complex networks.

Understanding wildcard masks can dramatically improve your efficiency in writing ACLs, implementing routing policies, and designing scalable networks. This topic is frequently encountered in certifications like CCNA, CCNP, and various Cisco exams, all of which DumpsArena provides high-quality study materials for.

Conclusion

Understanding how wildcard masks function and how to calculate them effectively is a vital skill for any network administrator. The ability to determine the correct wildcard mask, such as for matching networks 172.16.0.0 through 172.19.0.0, sets apart knowledgeable professionals from beginners. As demonstrated, the wildcard mask 0.3.255.255 efficiently matches the specified range and simplifies ACL configuration.