CIDR is the standard, scalable method for both designating and organizing IP addresses using variable length subnet masking to optimize packet routing efficiency over the Internet. In the CIDR standard, IP address blocks are represented using an IP address with a suffix, as in 214.100.48.00/20, where the suffix, /20, indicates the number of leading bits in the binary form of the IP address corresponding to the intended subnet.

The following examples show equivalent representations of the same subnet:

11010110.01100100.00111001.11010101 = 214.100.57.213/32

11010110.01100100.00111001.11010000 = 214.100.57.208/28

11010110.01100100.00111001.00000000 = 214.100.57.00/24

11010110.01100100.00110000.00000000 = 214.100.48.00/20

Using CIDR, network administrators have a great amount of flexibility in terms of defining the size of available IP address allocations. The basic formula for determining the size of a CIDR subnet is s=2^(n-32), where S = the number of available IP addresses and n = the CIDR suffix. The following table displays the correlation between the CIDR suffix (/n) and the number of available IP addresses, or hosts, for multiple, different CIDR suffixes.

In subnets defined to contain more than 2 IP addresses, typically the smallest address identifies the subnet to the rest of the network and the largest address is designated as the broadcast address for all addresses contained within the subnet.

As a simple example case of CIDR notation with respect to subnets, both 214.100.50.20 and 214.100.61.45 are in the subnet 214.100.00.00/16 because they both share the same sixteen leading bits, represented by the decimal digits 214.100. These two IP addresses also exist in an even smaller subnet, 214.100.48.0/20, as revealed when the two addresses are expressed in binary, as follows, where the twenty leading bits are identical:

11010110.01100100.00110010.00000100 = 214.100.50.04

11010110.01100100.00111101.00101101 = 214.100.61.45

11010110.01100100.00110000.00000000 = 214.100.48.0/20

In a subnet two IP addresses are reserved and cannot be assigned:

• The network address
• The broadcast address

Other IP addresses can be reserved for special use in IPAM so they will not be assigned, either by an administrator or DHCP, to other devices.

In SolarWinds IPAM, reserved IP addresses are indicated with a purple IP icon. See IPAM status icons for more information.

A subnet is any logical or physical subdivision of a network consisting of a collection of IP addresses for which some number of the leading address bits, commonly called an IP address routing prefix, are identical.

For example, as a simple case, both 214.100.50.20 and 214.100.61.45 are in the subnet 214.100.00.00/16, as they both share the same sixteen leading bits, represented by the decimal digits 214.100. Less obviously, these two IP addresses exist in an even smaller subnet, 214.100.48.00/20, as revealed when the two addresses are expressed in binary, as follows, where the twenty leading bits are identical:

214.100.50.04 = 11010110.01100100.00110010.00000100

214.100.61.45 = 11010110.01100100.00111101.00101101

11010110.01100100.00110000.00000000 = 214.100.48.00/20

Organizing your network using well-defined subnets can greatly increase the efficiency and minimize the bandwidth load on your network. At a basic level, assigning IP addresses to devices on your network in such a way that highly interactive devices reside within smaller or closer subnets reduces the amount of network traffic that must be routed over longer network distances. See Manage subnets in IPAM for more information about creating and managing subnets in SolarWinds IPAM.

A supernet is an element of network organization consisting of contiguous CIDR blocks, or subnets. In networks with well-defined subnets, network administrators are able to consolidate and limit IP traffic with supernets to optimize routing efficiency across a network. As an example, given the following two subnets, 222.22.12.0/24 and 222.22.10.0/24, 222.22.0.0/20 is a supernet, as shown in the following expansions:

222.22.12.0/24 = 11011110.00010110.00001100.00000000

222.22.10.0/24 = 11011110.00010110.00001010.00000000

222.22.0.0/20= 11011110.00010110.00000000.00000000

IPAM uses the term Transient to describe IP addresses that are dynamically assigned to devices. IP addresses designated as Transient may be assigned to any of the following types of devices:

• Devices that power on and off regularly, such as laptops or some user workstations
• Devices that enter and exit the network frequently, such as laptops on a wireless network
• Any device on a DHCP-enabled network

Transient scan intervals can be configured on a per subnet basis from the Edit Subnet window.

In IPAM, Transient IP addresses are indicated with a cyan colored IP icon. See IPAM status icons for more information.