What is CIDR?
Classless Inter-Domain Routing (CIDR) provides a more flexible and efficient means to allocate and specify Internet addresses. Before CIDR, IP addresses were divided into general classes where the first part of the IP address represented the network address (of 8, 16, or 24 bits) and the latter part represented the host address. Because the classes were of a fixed size, many assigned IP addresses went unused. Skip the details and find out how to get the CIDR notation for an IP address.
With CIDR, the IP address still has 32-bits but now has a network prefix (of 13-27 bits) indicating how many of the 32 bits are specific to the network address and how many are specific to the host (or machine) address.
For example:
- 216.109.0.0 /24 represents the 256 IP addresses 216.109.0.0 through 216.109.0.255
The "24" indicates that the first 24 bits (216.109.0) are associated with the network address with the remaining (8) bits associated with host (machine) address.
| IP in CIDR Notation | 216.109.0.0/24 |
| # of bits in Network Address | 24 of 32 |
| # of bits in Host Address | 8 of 32 |
| # of bits (8) in binary notation | 11111111 |
| Binary conversion to decimal | 255 |
| # of addresses | 0-255= 256 total |
- 216.109.0.0 /22 represents the 1024 IP addresses 216.109.0.0 through 216.109.3.255
The "22" represents that the first 22 bits are associated with the network address with the remaining 10 bits associated with the host (machine) address.
| IP in CIDR Notation | 216.109.0.0/22 |
| # of bits in Network Address | 22 of 32 |
| # of bits in Host Address | 10 of 32 |
| # of bits (10) in binary notation | 1111111111 |
| Convert to octets | 00000011.11111111 |
| Binary conversion to decimal | 4 . 255 |
| # of addresses | 4 × 0-255 (256) =1024 total |
In terms of IP blocking, CIDR allows you to specify an entire range of IP addresses by blocking at the network address level rather than individual IP addresses. While this can save time when you wish to block a network associated with a high-risk country or organization, you should be cautious about adding ranges with CIDR notation as there is a greater potential for blocking legitimate buyers if you make a mistake.
Finding the CIDR Notation for an IP Address:
You can typically get the CIDR notation for an IP address on a network or organization by performing a reverse DNS lookup. After looking up information for a single IP address, click the "see complete whois information" link. The popup window should display the CIDR range for the network on which the host originated (see figure 1)
| Note: Yahoo! provides the IP address blocking feature as is and without warranty of any kind to Yahoo! Merchant Solutions and Store merchants to be used at the merchants' discretion. Merchants should carefully review the Reverse DNS Lookup information provided before blocking any IP address, as IP addresses may be dynamically assigned to users by certain Internet Service Providers possibly blocking legitimate buyers and not ensuring the fraudster is blocked from ordering. Also, as IP addresses may be falsified, merchants are advised to use AVS and CVV responses to determine which orders to approve, reject, or follow up with the issuing bank. |
Learn more about CIDR:
- TechTarget—definition of CIDR
- Wikipedia—explanation of CIDR
See Also
- IP Blocking Overview
- Blocking an Individual IP Address
- Blocking a Range of IP Addresses
- Removing an IP Address from the Block List
- What is an IP Address?
- What will a customer see coming from a Blocked IP?
- How do I lookup information for an IP Address from a suspicious order?
- What are the signs of a suspect order?
- Can IP Addresses be spoofed or falsified?
- Is there a limit to the number of entries in the IP Block List?