Combine IPv4 or IPv6 CIDR blocks into the smallest supernet that covers them all. Useful for route summarisation and firewall rules.
Enter CIDR blocks above, one per line
A supernet (or aggregate) is a single CIDR block that covers two or more smaller subnets. Supernetting is the inverse of subnetting - instead of dividing one network into smaller pieces, you combine several networks into one larger covering block.
Route summarisation (BGP): Instead of advertising eight individual /24 routes to peers, you advertise a single /21 supernet. Fewer routes mean faster convergence and smaller routing tables across the internet. This is one of the primary reasons BGP table growth has been manageable despite the explosion of connected networks.
Firewall rules: Allowing access from 10.0.0.0/24 and 10.0.1.0/24 can often be simplified to one rule for 10.0.0.0/23, reducing rule count and complexity.
IP address planning: Verify that a set of planned subnets fits cleanly within a parent allocation block before committing to a design.
The calculator finds the smallest single prefix that contains the leftmost network address and rightmost broadcast address across all input CIDRs. It XORs those two boundary addresses - the position of the highest differing bit determines the covering prefix length. The result is then aligned to the correct network boundary for that prefix.
Efficiency is the fraction of the supernet's address space actually covered by your input CIDRs. If you aggregate 192.168.1.0/24 and 192.168.2.0/24, the smallest covering supernet is 192.168.0.0/22 (1,024 addresses). Your two /24s cover 512 of those - 50% efficiency. The remaining 512 addresses (192.168.0.x and 192.168.3.x) are in the supernet but not in your input. 100% efficiency means your inputs exactly fill the supernet with no gaps.
For BGP route summarisation, 50% efficiency is often acceptable - the routing benefit outweighs the wasted address space. For firewall rules, gaps (addresses in the supernet but not in your input) may represent addresses you did not intend to permit - check the efficiency note before applying the supernet as a rule.