CIDR Aggregator

CIDR aggregation combines multiple adjacent IP prefixes into a shorter prefix when their binary boundaries align. For example, two adjacent /25 networks can form one /24 if they cover the full address range and begin on the correct boundary. The aggregate route represents the same span with fewer entries.

This is route summarization. It reduces routing table size, simplifies firewall rules, and makes address plans easier to communicate. Aggregation is only valid when the summarized block contains exactly the intended networks or when the operator is willing to route the extra included space the same way.

CIDR prefixes are binary ranges. Aggregation works by finding shared leading bits among addresses. The more leading bits two networks share, the more specific the aggregate. If networks differ before the desired prefix boundary, they cannot be summarized without including unrelated addresses.

Dotted-decimal addresses can make adjacent ranges look simpler than they are. Binary alignment decides. A /23 must start on an even third-octet boundary, for instance, because it spans two /24 blocks. Misaligned aggregation creates ranges that do not match the actual address space.

Summarization improves routing stability. If internal subnets flap, an upstream aggregate can remain stable, limiting churn outside the local domain. It also reduces cognitive load: a team can reason about one site prefix instead of dozens of smaller VLAN prefixes.

Firewalls and access lists can also benefit from aggregates, but only when policy is uniform across the summarized range. If two subnets need different treatment, summarizing them in a rule may grant or deny too much. Routing convenience should not override security boundaries.

The main risk is over-aggregation. A summary that includes unassigned or differently routed space can send traffic to the wrong place or create black holes. Another risk is hiding design problems: summarization is easier when address allocation was planned hierarchically from the beginning.

Good CIDR aggregation checks adjacency, alignment, coverage, and policy. The shortest possible prefix is not always the best operational prefix. The right summary is the one that matches both the binary math and the network design.