# What is CIDR?

CIDR, short for Classless Inter-Domain Routing, is a replacement for the older Classful network addressing architecture that was in use since 1981. CIDR was introduced in 1993 to address the issue of IP address shortage.

The CIDR method divides the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size, i.e. number of hosts for unicast addresses (classes A, B, C), or a multicast network (class D). The fifth class (E) address range is reserved for future or experimental purposes. Now, IPv6 is fast replacing IPv4 allowing huge expansion for more users and devices on the internet.

**Maskbit:**

Maskbits in CIDR allow you to discover specific IP ranges within the network.

**Example:** **/24** is the Maskbit for the Subnet **255.255.255.0**, i.e. **192.168.0.1/24 =** Range of IPs from **192.168.0.1** to **192.168.0.255**.

| Subnet | CIDR Maskbit | Total IPs in Range | Number of Class C networks |
|---|---|---|---|
| 255.255.255.255 | /32 | 1 | 1/256th |
| 255.255.255.240 | /28 | 16 | 1/16th |
| 255.255.255.0 | /24 | 256 | 1 |
| 255.255.0.0 | /16 | 65,536 | 256 |

## To discover a complete network (CIDR/Subnet based discovery):

For OpManager versions 125174 and above:

1. Go to **Settings** -> **Network Discovery** -> **New Discovery.**
2. Select the **Subnet** tab.
3. Network IP: Specify the Network IP to be discovered.
4. Select the [required Credentials](https://www.manageengine.com/network-monitoring/help/add-credentials.html)
5. Click on **Discover** and OpManager will direct you to the **'Discovered Devices'** page.
6. **Approve** or **Ignore** the discovered devices by clicking on the respective options. The approved devices will be added to the OpManager inventory and monitored. The ignored devices will be removed from the queue of discovered devices and restricted from future addition.

![CIDR in OpManager: Network discovery](https://www.manageengine.com/network-monitoring/faq/images/cidr-2.PNG)

For OpManager versions below 125174:

1. Go to **Settings** -> **Discovery** -> **Discovery Profile.**
2. Select the **CIDR** option.
3. Network IP: Specify the Network IP to be discovered.
4. Discovery Credentials: Select the [configured Credentials](https://www.manageengine.com/network-monitoring/help/add-credentials.html) required for the discovery process.
5. Navigate to the next page and configure Discovery rules, associate and discover interfaces and schedule the discovery process if required. Click on **Save and Execute** at the end to initiate discovery.

![CIDR in OpManager: Discovery - input](https://www.manageengine.com/network-monitoring/faq/images/cidr_1.PNG)

## Frequently asked questions:

1. [How to discover devices and interfaces in OpManager?](https://www.manageengine.com/network-monitoring/help/discover-networks.html)
2. [How to add a new credential?](https://www.manageengine.com/network-monitoring/help/add-credentials.html)
3. [Can I add a non-admin user account for WMI?](https://www.manageengine.com/network-monitoring/kb/adding-non-admin-user-for-wmi.html)
4. [Can I add devices using SSH?](https://www.manageengine.com/network-monitoring/help/adding-devices-using-ssh.html)
5. [Why are devices discovered as 'Unknown'?](https://www.manageengine.com/network-monitoring/faq/devices-discovered-as-unknown-devices-opmanager.html)
6. [What are devices supported in OpManager?](https://www.manageengine.com/network-monitoring/faq/all-devices-supported.html)
7. [Why are interfaces not discovered for my device?](https://www.manageengine.com/network-monitoring/faq/interfaces-not-discovered.html)
8. [How can I associate device templates to a newly discovered device?](https://www.manageengine.com/network-monitoring/help/associate-device-template.html)
9. [How to associate monitors to the discovered device?](https://www.manageengine.com/network-monitoring/help/adding-custom-monitors.html)
10. [What is a discovery filter?](https://www.manageengine.com/network-monitoring/help/discovery-filter.html)
11. [How do I configure a new discovery rule?](https://www.manageengine.com/network-monitoring/help/configuring-discovery-rule-engine.html)
12. [How to manage or unmanage a device?](https://www.manageengine.com/network-monitoring/help/managing-and-unmanaging-a-device.html)
13. [How to configure device dependencies?](https://www.manageengine.com/network-monitoring/help/configuring-device-dependencies.html)
14. [How can find the missing VMs and hosts?](https://www.manageengine.com/network-monitoring/faq/find-missing-vms-and-hosts-opmanager.html)
15. [How can I discover a VLAN?](https://www.manageengine.com/network-monitoring/faq/vlan-discovery-in-opmanager.html)
16. [How does availability monitoring work in OpManager?](https://www.manageengine.com/network-monitoring/availability-monitoring.html)
17. [What are the basic system requirements to install and use OpManager?](https://www.manageengine.com/network-monitoring/help/hardware-and-software-requirements.html)
18. [How can I troubleshoot issues with device discovery?](https://www.manageengine.com/network-monitoring/help/add-device-failure-messages.html)
19. [How can I troubleshoot issues with VCenter/ESX discovery?](https://www.manageengine.com/network-monitoring/faq/troubleshooting-problems-in-vcenter-discovery.html)