CIDR Aggregation Checker
Can aggregate
192.168.0.0/24
How it works
CIDR aggregation (also called route summarization or supernetting) combines multiple smaller CIDR blocks into a single larger prefix that encompasses all of them. Aggregation reduces the size of routing tables, decreasing memory usage on routers and improving BGP convergence time. This tool checks whether a set of CIDR blocks can be aggregated and computes the minimal covering supernet.
**Why aggregation matters** The global BGP routing table has grown to over 900,000 IPv4 prefixes and 180,000 IPv6 prefixes. Poorly aggregated allocations contribute to table bloat. Proper aggregation: an ISP with 192.168.0.0/24 and 192.168.1.0/24 can advertise a single 192.168.0.0/23 route (the /23 covers both /24s). Unaggregated: two separate BGP advertisements; aggregated: one advertisement with half the routing state.
**Aggregation conditions** Two /N blocks can be aggregated into a single /(N−1) if they are adjacent and their network addresses differ only in the (N)th bit. 192.168.0.0/24 and 192.168.1.0/24 share the first 23 bits — aggregate to 192.168.0.0/23. But 192.168.1.0/24 and 192.168.2.0/24 differ in the 22nd bit — they cannot form a clean /23 aggregate (the covering supernet would be 192.168.0.0/22, unnecessarily including .0.0/24 and .3.0/24).
**Longest prefix match** Routers use longest prefix match when looking up a destination: a /28 route is preferred over a /24 covering the same addresses. Aggregation trades specificity for size — aggregate only when all sub-prefixes within the supernet are reachable via the same next-hop.
Frequently Asked Questions
- Two /24s can aggregate to a /23 if they are adjacent and differ only in the 24th bit. Check: 192.168.0.0/24 and 192.168.1.0/24. Convert the third octet to binary: 0 = 00000000, 1 = 00000001. They differ only in the last bit of the third octet (bit 24). The /23 aggregate: 192.168.0.0/23 (set bit 24 to 0, keep bits 1–23 as the prefix). Verify: the /23 covers 192.168.0.0–192.168.1.255, which includes both original /24s.
- Every network prefix in the BGP routing table requires memory in every router's RIB (Routing Information Base) and processing during path selection. The global IPv4 BGP table exceeded 900,000 entries in 2024. Without aggregation, each /24 (254 hosts) is advertised separately — a /16 containing 256 /24s would add 256 entries. Aggregated to one /16 entry, only 1 entry is needed. Large ISPs aggregate their customer allocations at their AS boundary before propagating to the global DFZ (Default-Free Zone). Fragmented routing tables also slow BGP convergence after failures.
- The IANA default route filter minimum for IPv4 is /24 — most BGP peers filter out prefixes longer than /24 (more specific than a /24). This is a soft convention maintained by most networks to prevent BGP table bloat. For IPv6, the convention is /48. If you advertise a /25 or /28, most upstream providers will silently drop it. To ensure global reachability, always announce at least a /24 (IPv4) or /48 (IPv6) aggregate, even if your actual allocation is more specific.
- Before 1993, IP addresses were assigned in rigid classes: Class A (/8, 16M hosts), Class B (/16, 65K hosts), Class C (/24, 254 hosts). A company needing 500 hosts received a Class B (65K addresses) — wasting 64.5K addresses. The internet routing table grew unsustainably as Class Cs proliferated. CIDR (RFC 1519, 1993) introduced variable-length subnet masks, allowing any prefix length /0–/32. A company needing 500 hosts receives a /23 (1022 hosts) instead of a /16. CIDR also enabled route aggregation: multiple /24s could be summarized into a single /16, shrinking the routing table.