Subnet Calculator

IPv4 subnetting with instant results, binary view and multi-format masks.

Input

Accepts "IP/CIDR", "IP mask" or "IP CIDR".

Output

Type an IP and a CIDR to see the subnet details.

How IPv4 subnetting works

Every IPv4 address is a 32-bit number, typically written in dotted-decimal notation (e.g. 192.168.1.0). A subnet mask — or its compact CIDR equivalent — divides those 32 bits into a network portion and a host portion. The boundary between the two is what defines a subnet.

CIDR notation (Classless Inter-Domain Routing) expresses that boundary as a prefix length appended to the address: 192.168.1.0/24 means the first 24 bits identify the network, leaving 8 bits for host addresses. That yields 28 − 2 = 254 usable hosts (the network address and broadcast address are reserved). Extending the prefix to /26 borrows 2 more bits from the host field, splitting the original /24 into four equal /26 blocks of 62 usable hosts each — a common operation when segmenting a LAN by department or VLAN.

Network engineers use subnetting daily: to size VLANs correctly, to write ACL wildcard masks, to summarise routes for OSPF or BGP, or simply to verify that two hosts are on the same subnet. A reliable CIDR calculator removes the mental arithmetic and eliminates transcription errors in production configs.

This subnet calculator accepts input as IP/CIDR, IP mask or IP CIDR (space-separated). Results update instantly on each keystroke and include the network address, broadcast address, first and last usable hosts, subnet mask in all four common formats (dotted decimal, hex, CIDR, wildcard), and a colour-coded binary view that makes the network/host boundary visible at a glance. Need to carve the result into smaller subnets of unequal size? Send it directly to the VLSM Planner.

Common use cases

  • CCNA exam practice — verify manual IPv4 subnetting calculations instantly.
  • ACL design — derive the wildcard mask needed for access-list or ip access-group statements.
  • Route summarisation — confirm that a supernet correctly covers a range of child prefixes.
  • Addressing plans — check whether a proposed subnet fits within an existing allocation without overlap.
  • Cisco config review — cross-check interface addresses against their configured masks quickly.

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