All the IP addresses that you officially allow to be used for sending emails on your behalf are listed in an SPF record. When domain owners have to mention multiple IP addresses, they often use the CIDR (Classless Inter-Domain Routing) notation as it’s a way of representing IP address ranges in a compact manner.
Using CIDR notation in an SPF record allows you to specify large blocks of IP addresses easily and in an organized manner, without having to enlist individual addresses separately. This approach is particularly useful for organizations with an extensive email system, large networks, and dynamic IP ranges.
However, sometimes, a CIDR notation error occurs in an SPF record when the specified CIDR block is incorrect or invalid. This blog discusses why a CIDR error prompts and how it can affect the working of SPF for a domain.
What is CIDR Notation?
CIDR notation is a method to represent ranges of IP addresses and is generally used in internet routing and networking. Using this method, you can efficiently allocate and manage IP addresses.
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In CIDR notation, an IP address is followed by a forward slash (/) and a number, which tells how many bits are fixed in the network portion of the address.
For example- 192.168.1.0/24
In this example, 192.168.1.0 is the IP address, and the first 24 bits are fixed as the network portion of the address.
Experts prefer the CIDR notation method over older methods like classful addressing, as the former offers more flexibility in allocating IP addresses. Moreover, CIDR notation is not dependent on prespecified address classes.
The flexibility that comes with CIDR notation helps use IPv4 address space easily, which has gone scarce and taken up by its successor, IPv6, due to the growth of the internet. Another plus point is that it facilitates hierarchical routing, a method in which a network is divided into multiple levels of hierarchy, each with its own routing algorithm. The hierarchical routing method is appreciated because of its efficiency in routing traffic across the internet.
Why Does CIDR Notation Error Occur?
CIDR notation error occurs if you add an incorrect or invalid CIDR block prompting out of the following possible reasons-
Syntax Error
CIDR notation follows a specific syntax that includes an IP address followed by a forward slash and a number. The number is called the prefix length, which basically represents how many bits are fixed in the network portion of the address. A CIDR notation error would occur if the syntax misses the prefix length or uses an invalid character.
Invalid IP Address Range
The specified CIDR block might not represent a valid range of IP addresses. For example, specifying an IP address that falls outside the valid range for IPv4 (0.0.0.0 to 255.255.255.255) would result in an error.
Incorrect Prefix Length
The prefix length in CIDR notation determines the number of significant bits in the network portion of the IP address. An error occurs if the prefix length is too large or too small for the specified IP address range.
Overlap or Conflict
An SPF record can have multiple CIDR blocks, but an error will likely prompt if there is overlap or conflict between the specified CIDR blocks, resulting in ambiguity in SPF validation.
How does the CIDR Notation Error Affect SPF?
A CIDR notation error can affect the working of an SPF record, leading to false positives or negatives. DMARC, which is another efficient email authentication protocol, is based on the results of SPF and DKIM authentication checks. So, if there is a problem in SPF, it’s likely to influence DMARC’s working and results as well, causing a disruption in the overall email security process. Here are some of the issues that you can expect because of CIDR notation error-
Email Delivery Issues
If you mention a wrong or invalid CIDR block range, genuine emails sent from your domain will be flagged as suspicious and won’t be placed in the recipients’ inboxes. You may not even realize this until a recipient intimates you of not receiving a supposed email from you.
Security Vulnerability
If you add an incorrect CIDR range, then an unauthorized entity will be able to send emails from your domain, and since the IP address will be the part of your SPF record, the recipients’ servers would identify them as legitimate. This can lead to successful phishing and spoofing attempts.
Policy Ineffectiveness
The purpose of SPF records is to specify which IP addresses are authorized to send emails on behalf of a domain. CIDR notation errors can render the SPF policy ineffective by allowing unauthorized senders or blocking legitimate ones.
Final Words
Using CIDR notation is a suggested way for mentioning IP addresses, especially if they belong to a dynamic range. However, with the use of this method, an added responsibility comes of ensuring that you include only valid and correct IP ranges. If your SPF record is having some sort of error and you are failing to identify one, then reach out to us. There’s a possibility that you might be overlooking IP ranges that don’t belong to you.
