What is CIDR?

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing traffic that replaced the old fixed-size address classes with variable-length subnet prefixes. A CIDR block is written as an address followed by a slash and a prefix length – for example, 172.16.4.0/25 – sizing address ranges to actual need instead of rigid class boundaries.

Key takeaways

• CIDR replaces fixed address classes with variable-length prefixes.
• Notation pairs an IP address with a slash and a prefix length, e.g. /24.
• The prefix length sets how many addresses a block contains, e.g. /30.
• CIDR also enables route summarization, shrinking global routing tables.
• Versa configuration screens use CIDR notation for subnets, VLANs, and tunnels.

On this page: Why it matters · How it works · Types · CIDR & Versa · FAQ

Why is CIDR important?

Before CIDR, IP addresses were assigned in fixed classes – Class A, B, and C – giving every organization a block of roughly 16 million, 65,000, or 256 addresses, nothing in between. An organization needing 500 addresses wasted most of a Class B block or consumed several Class C blocks, and each block became a separate router-table entry. By the early 1990s this waste, plus explosive growth, threatened to exhaust the IPv4 address space and overwhelm routers.

CIDR, introduced in 1993, solved both problems at once. It let operators carve blocks to the size actually needed, and aggregate many small blocks into a single route advertisement – slowing routing-table growth. That combination is why CIDR still underlies how ISPs, cloud providers, and enterprise networks describe every address range today, from a firewall rule to a VPC subnet.

What is CIDR?

CIDR notation pairs an IP address with a prefix length: the number of leading bits, out of 32 for IPv4, that form the fixed network portion of the address. The remaining bits identify hosts within that block. A shorter prefix leaves more host bits and a larger block; a longer prefix leaves fewer host bits and a smaller one. 172.16.4.0/25 means the first 25 bits are fixed as the network, leaving 7 host bits – 128 addresses, equivalent to a subnet mask of 255.255.255.128.

The same idea works in reverse for routing: instead of advertising many small, contiguous blocks separately, a router can advertise one larger block that covers all of them – a technique called supernetting or route summarization. That is what keeps backbone routing tables from growing at the same pace as the number of networks connected to the internet.

Types of CIDR blocks

CIDR prefix lengths map to conventional uses. Large blocks with short prefixes (/8 through /15) are typically ISP or regional registry allocations. Mid-size blocks (/16 through /23) commonly size an enterprise site or a cloud VPC. /24 is the classic LAN subnet – 256 addresses, matching the old Class C size and still the most common default. Smaller blocks (/25 through /30) size VLANs, DHCP pools, and point-to-point links; /30 is standard for a link with exactly two usable addresses, such as a tunnel interface. /32 identifies a single host route. IPv6 uses the same notation at a larger scale, with /64 the conventional subnet size.

CIDR & Versa

Versa configuration screens use CIDR notation wherever an address range needs specifying. LAN and VLAN interfaces take an IP prefix and length in this format – the Titan Portal documents fields such as “IP Address/Mask” and DHCP “IP Subnet” requiring a valid CIDR value, for example 172.16.4.0/25, and tunnel interfaces likewise take a local IP address in CIDR format.

CIDR blocks also govern cloud gateway deployments. When integrating a Versa SASE gateway with an AWS VPC, the VPC and its subnets are each assigned a CIDR block, and a deployed vCSG’s LAN and WAN interfaces must be addressed from within it. Tunnel configuration references the inside and outside CIDR ranges the cloud provider returns when the tunnel is created.

Frequently asked questions

What does the CIDR prefix length actually mean?

It is the number of leading bits in the address that form the fixed network portion – out of 32 total for IPv4. A /24 fixes the first 24 bits, leaving 8 host bits and 256 addresses; a /30 fixes 30 bits, leaving 4 addresses, typically used for point-to-point links.

What is the difference between a subnet mask and CIDR notation?

They express the same thing two ways. A subnet mask like 255.255.255.0 and a CIDR prefix like /24 both mean the first 24 bits are the network portion. CIDR notation is shorthand that avoids writing out the full dotted-decimal mask.

What is route summarization?

Also called supernetting, it is combining several contiguous smaller CIDR blocks into one larger block and advertising that single block as a route. It reduces the number of entries a router must hold, which is one of CIDR’s core purposes.

Does CIDR apply to IPv6?

Yes. IPv6 addresses use the identical address/prefix-length notation, just at a much larger scale – a /64 is the conventional subnet size, compared to a /24 being conventional for IPv4 LANs. Address exhaustion was never IPv6’s motivation, but CIDR’s prefix arithmetic still governs how its subnets are sized and routed.

Why did CIDR replace classful addressing?

Classful addressing only offered fixed block sizes – Class A, B, or C – which wasted addresses whenever an organization’s actual need fell between them, and produced unmanageably large routing tables as the internet grew through the early 1990s. CIDR let allocations match actual need and let routes aggregate, easing both problems at once.

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