IPv4 Subnet, Cloud VPC & CIDR Calculator

Calculate network boundaries, usable host ranges, Cisco wildcard masks, and AWS / Azure / GCP reserved VPC capacities in real time.

Subnet Calculation Results (click any value to copy)

Infrastructure-as-Code & Router ACL Snippets

How IPv4 CIDR & Cloud VPC Subnet Reservations Work

Classless Inter-Domain Routing (CIDR), standardized in RFC 4632, replaces legacy Class A, B, and C addressing with flexible prefix lengths written as /0 through /32. The prefix number defines how many leading bits of a 32-bit IPv4 address identify the network portion, while the remaining 32 - prefix bits identify individual hosts inside that subnet.

On traditional Ethernet switches and routers, two addresses are reserved per block—the network identifier (all host bits 0) and the directed broadcast address (all host bits 1)—leaving 2^(32 - prefix) - 2 assignable hosts. However, major public clouds reserve additional infrastructure addresses inside every VPC subnet: AWS VPC and Azure VNet reserve 5 IPs (.0 network, .1 VPC router, .2 cloud DNS, .3 future reservation, and broadcast), while Google Cloud reserves 4 IPs. Under RFC 3021, /31 prefixes support 2-host point-to-point router links, and /32 pinpoints a single host route.

Network engineers configuring Cisco IOS access control lists (ACLs) or OSPF statements also rely on the wildcard mask, the exact bitwise inverse of the subnet mask. Need to partition one parent CIDR block across differently sized tiers? Launch our VLSM Subnet Splitter.