Using RFC1918 Ranges in Prefix Lists

The dynamic world of Information Technology is chock-full of terms and practices that can sometimes be challenging to decipher. One concept that often perplexes network professionals is using RFC1918 ranges in prefix lists. This method is frequently employed to blanket all possible internal addresses in a network, even if some of those addresses are not currently in use. In this blog post, we’ll explore what it means to use RFC1918 ranges in this manner, why it might be useful, and the potential drawbacks of such an approach.

What does it mean to use RFC1918 ranges in prefix lists?

When network engineers use RFC1918 ranges in their prefix lists, they cast a wide net to capture all conceivable internal addresses. RFC1918 defines the private IPv4 address spaces that can be used within any private network, including:

  • – (
  • – (
  • – (

By employing these ranges, network engineers create a catch-all or blanket that covers all potential internal routes without specifically listing each one. This approach can be beneficial in a multitude of scenarios. For example, it might be used to direct all internal traffic to a particular interface or apply a specific policy to all potential internal traffic. 

In principle, we could say, “This rule applies to all our internal traffic, regardless of its origin or specific route.” However, it’s a bit of a simplification. A more accurate statement would be, “This rule applies to all our internal traffic unless a more specific route applies.” 

The intent of creating such a broad RFC1918 route is typically to provide a “catch-all” that will handle any internal traffic not covered by a more specific route. It’s a way of ensuring that all traffic has a valid path, even if no specific route has been configured for it. 

However, it is important to note that this practice does not impact traffic originating from outside the network, as external networks cannot directly route to these private addresses.

Benefits of Using RFC1918 Ranges in Prefix Lists

1. Simplicity and Efficiency: One of the most significant advantages of this method is its simplicity. It makes network configuration and management much easier since all possible routes can be included in one statement, reducing the complexity and size of routing tables.

2. Scalability: As your network grows and evolves, you don’t have to update your routing policy each time you add a new subnet. The blanket route you’ve created encompasses all possible internal addresses, providing an element of future-proofing.

3. Uniform Control: This approach allows a network engineer to apply consistent policies to all internal traffic. This means they can uniformly manage and control how their internal traffic is handled unless a more specific route applies.

Potential Drawbacks of Using RFC1918 Ranges in Prefix Lists

While this method offers significant benefits, it’s not without its potential drawbacks:

1. Lack of Precision: When you apply a blanket rule to all internal addresses, you give up a certain degree of precision. If your network has different subnets that require distinct routing policies, this approach may not be appropriate.

2. Potential for Misconfiguration: Without a deep understanding of what your blanket route is doing, you might inadvertently misroute traffic or apply incorrect policies. This can lead to operational inefficiencies and potential network vulnerabilities.

3. Increased Complexity in Troubleshooting: This method may lead to the creation of routing entries for networks that don’t actually exist in your topology. This could make troubleshooting more complex and lead to inefficient use of router resources.

4. Security Concerns: Broad stroke routing can potentially introduce security issues. If precise control is lost and the network is ever compromised, malicious actors might find it easier to navigate through it.

In conclusion, using RFC1918 ranges in prefix lists to blanket all possible internal addresses is a strategy that offers both significant advantages and potential pitfalls. When deploying this method, it’s critical to carefully evaluate your network’s unique requirements and constraints to ensure that it delivers the benefits you seek without introducing unnecessary risks.