CIDR (Classless Inter-Domain Routing)

Understanding CIDR

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing traffic. Unlike older class-based addressing (Class A, B, C), CIDR enables more flexible and efficient allocation of IP addresses using prefix notation, such as:

192.168.1.0/24

In CIDR, the number following the slash (e.g., /24) is the network prefix, which determines how many IP addresses are included in the subnet range.

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Why is CIDR Important?

When creating a subnet, you must define how many IP addresses should be available for resources within that subnet. CIDR notation gives you precise control over subnet sizing, allowing you to balance between:

• Small subnets – e.g., /30 for limited usage scenarios

• Large subnets – e.g., /16 for high-scale deployments

Choosing the right CIDR size ensures efficient use of IP space while minimizing waste.

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CIDR Size Table

CIDR Notation	Number of IPs	Usable IPs	Subnet Mask	Example Range
/32	1	0	255.255.255.255	192.168.1.1
/30	4	2	255.255.255.252	192.168.1.0 – 192.168.1.3
/29	8	6	255.255.255.248	192.168.1.0 – 192.168.1.7
/28	16	14	255.255.255.240	192.168.1.0 – 192.168.1.15
/27	32	30	255.255.255.224	192.168.1.0 – 192.168.1.31
/26	64	62	255.255.255.192	192.168.1.0 – 192.168.1.63
/24	256	254	255.255.255.0	192.168.1.0 – 192.168.1.255
/23	512	510	255.255.254.0	192.168.0.0 – 192.168.1.255
/22	1,024	1,022	255.255.252.0	192.168.0.0 – 192.168.3.255
/21	2,048	2,046	255.255.248.0	192.168.0.0 – 192.168.7.255
/20	4,096	4,094	255.255.240.0	192.168.0.0 – 192.168.15.255
/16	65,536	65,534	255.255.0.0	192.168.0.0 – 192.168.255.255

Note: The number of usable IPs is always 2 less than the total IPs — one is reserved as the network address and the other as the broadcast address.

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Choosing the Right CIDR Block

When selecting a CIDR block, use the following general guidance:

• Use /30 or /29 for small setups (e.g., 1–6 VMs)

• Use /27 or /26 for medium setups (e.g., 30–60 VMs)

• Use /24 or /23 for large-scale environments

For most users, /28 or /27 is a good starting point unless larger address space is explicitly required.