The abbreviation CIDR stands for "Classless Inter-Domain Routing." It is a method for distributing and controlling IP addresses in computer networks. The traditional IP address scheme, known as classful addressing, divided IP addresses into fixed-size classes (Class A, B, and C) based on their initial bits. However, this approach led to inefficient allocation of IP address space.

With CIDR, IP addresses are represented by a prefix, a slash (/), and a number, which indicates the number of bits utilized for the network portion of the address, allowing for precise and effective allocation of IP address blocks.

In CIDR notation, for instance, an IP address block might be written as "192.168.1.0/24," where the "/24" denotes that the network address's first 24 bits are used. Network administrators can assign smaller or larger subnets as needed by using the leftover bits for host addresses, which reduces the waste of IP addresses and enables better control of network resources.

ThreatNG External Attack Surface Management (EASM), Digital Risk Protection (DRP), and Security Ratings use various techniques to discover and assess CIDR (Classless Inter-Domain Routing) blocks within an organization's network. Understanding CIDR blocks is crucial as it helps organizations identify potential attack vectors, evaluate risks, and implement targeted security measures to safeguard their assets and sensitive information from external threats.