CIDR Aggregator & Deaggregator
Merge multiple CIDR blocks into the fewest possible supernets, or expand a large prefix into smaller subnets of any size. A free tool by IPv4Center.
What Is CIDR Aggregation?
CIDR aggregation (also called route summarization or supernetting) is the process of combining multiple contiguous IP address blocks into a single, larger CIDR block. This reduces the number of entries in routing tables, improving router performance and simplifying network management. For example, four consecutive /24 networks (192.168.0.0/24 through 192.168.3.0/24) can be aggregated into a single /22 (192.168.0.0/22).
How Does CIDR Aggregation Work?
The aggregation algorithm sorts all input CIDR blocks numerically, then iteratively checks if adjacent blocks can be merged. Two blocks can merge if they are the same size, their network addresses differ only in the bit immediately above their prefix length, and the result is a valid CIDR boundary. The process repeats until no further merges are possible, producing the minimum set of CIDR blocks that covers exactly the same address space.
How Does CIDR Deaggregation Work?
Deaggregation (also called prefix splitting or breaking down) takes a large CIDR block and divides it into smaller, more specific subnets. For example, a /16 block contains 65,536 addresses and can be split into 256 × /24 subnets (each with 256 addresses), or 64 × /22 subnets, or 4 × /18 subnets. This is useful when you need to announce more specific routes for traffic engineering, assign subnets to customers, or distribute address space across multiple locations.
Common Use Cases
BGP Route Optimization
Network operators aggregate prefixes to reduce BGP routing table size, improving convergence time and reducing memory consumption on edge routers.
Firewall & ACL Management
Combine multiple allow/deny rules into fewer aggregated entries, making firewall configurations cleaner and more efficient.
IP Address Planning
Deaggregate large allocations into smaller blocks for assignment to departments, branch offices, or customers.
RIR Allocation Management
Manage IP address allocations from Regional Internet Registries (RIPE, ARIN, APNIC) by splitting and tracking sub-assignments.
Understanding CIDR Notation
CIDR (Classless Inter-Domain Routing) notation represents an IP address block using a base address followed by a slash and prefix length. The prefix length indicates how many leading bits define the network portion. A /24 means the first 24 bits are the network part, leaving 8 bits for host addresses (2^8 = 256 addresses total).
Unlike the older classful addressing system (Class A, B, C), CIDR allows network boundaries at any bit position from /0 to /32. This flexibility enables efficient allocation of address space — a company needing 500 addresses can receive a /23 (512 addresses) instead of wasting a full Class B /16 with 65,536 addresses.
Common CIDR blocks in practice: /32 = single host, /31 = point-to-point link (RFC 3021), /30 = 2 usable hosts (legacy PTP), /24 = 254 usable hosts (standard LAN), /22 = 1,022 usable hosts, /20 = 4,094 hosts, /16 = 65,534 hosts, /8 = 16.7 million hosts.
Route Summarization in BGP
Route summarization (supernetting) is critical for Internet scalability. The global BGP routing table currently holds over 1 million prefixes. Without aggregation, every deaggregated prefix would need to be stored and processed by every Internet router, consuming memory and CPU resources.
When an ISP receives a contiguous /16 allocation from a RIR, they can announce the entire /16 as a single BGP prefix rather than 256 individual /24 routes. Downstream customers may announce more-specific /24 routes for traffic engineering (e.g., multi-homing), but upstream providers should aggregate these where possible to keep the global table compact.
Best practice: Always aggregate to the maximum extent possible at your network boundary. Use prefix-lists and route-maps to prevent unnecessary deaggregation from propagating to upstream peers. Many IXPs and tier-1 providers filter prefixes longer than /24 for IPv4, meaning deaggregated /25 or smaller blocks may not propagate globally.
IP Address Space Management
Efficient IP address management requires both aggregation and deaggregation skills. When planning a new data center deployment, start with your total address requirement, round up to the nearest CIDR boundary, then deaggregate into functional subnets: management networks (/28 or /27), server VLANs (/24 or /23), DMZ segments (/26), and point-to-point router links (/31).
For organizations managing IP space from RIR allocations, maintaining clean aggregation boundaries simplifies RPKI ROA (Route Origin Authorization) management, geofeed records, and abuse handling. A well-aggregated address plan ensures that each announced prefix maps cleanly to organizational or geographic boundaries.
When transferring or leasing IPv4 address space, understanding deaggregation is essential. A /16 block being sold can be partitioned into /20 or /22 chunks for multiple buyers. Each resulting prefix must align on proper CIDR boundaries — you cannot arbitrarily split a /16 into differently-sized pieces without ensuring each piece starts at a valid network address for its prefix length.