What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation combines an IP address with a prefix length to define a network block. The prefix length (e.g., /24) indicates how many bits are used for the network portion. For example, 192.168.1.0/24 means the first 24 bits are the network, leaving 8 bits for host addresses — supporting 254 usable hosts.

What is the difference between a subnet mask and a wildcard mask?

A subnet mask uses 1s in the network portion and 0s in the host portion (e.g., 255.255.255.0 for /24). A wildcard mask is the inverse — 0s where the subnet mask has 1s and 1s where it has 0s (e.g., 0.0.0.255 for /24). Wildcard masks are commonly used in Cisco ACLs and OSPF routing configurations.

How do I calculate the number of usable hosts?

For a /n prefix, total hosts = 2^(32-n). Usable hosts = total hosts - 2, because the first address is the network address and the last is the broadcast address. Exception: /31 networks (used for point-to-point links) and /32 (single host) have all addresses usable under RFC 3021.

What are private IP address ranges?

Private IP ranges defined by RFC 1918 are: 10.0.0.0/8 (10.x.x.x), 172.16.0.0/12 (172.16.x.x to 172.31.x.x), and 192.168.0.0/16 (192.168.x.x). These are not routable on the public internet and are used for local networks, corporate intranets, and home routers.

What is a broadcast address?

The broadcast address is the last address in a subnet range. Packets sent to this address are delivered to all hosts in the subnet. It is formed by setting all host bits to 1 in the network address. For example, in 192.168.1.0/24, the broadcast address is 192.168.1.255.

サブネット計算機

IPアドレスとCIDR表記からサブネット情報を計算します。ネットワークアドレス、ブロードキャスト、ホスト範囲、サブネットマスクなどを確認できます。

IPアドレス / CIDR

CIDRプレフィックスを含むIPアドレスを入力してください（例: 192.168.1.0/24）

ネットワークアドレス	192.168.1.0
サブネットマスク	255.255.255.0
ブロードキャストアドレス	192.168.1.255
最初の使用可能ホスト	192.168.1.1
最後の使用可能ホスト	192.168.1.254
ワイルドカードマスク	0.0.0.255
総ホスト数	256
使用可能ホスト数	254
IPクラス	C
アドレスタイプ	プライベート

バイナリ表現

IPアドレス: 11000000.10101000.00000001.00000000

サブネットマスク: 11111111.11111111.11111111.00000000

ネットワーク: 11000000.10101000.00000001.00000000

使い方

1Type an IP address with its CIDR prefix into the input field — for example, 192.168.1.0/24. Or click one of the example buttons below the field to auto-fill a common network.
2The calculator instantly shows the network address, subnet mask, broadcast address, first and last usable hosts, total and usable host counts, IP class, and whether it is a private or public range.
3Check the Binary Representation section at the bottom to see the IP address, subnet mask, and network address in dotted binary notation — useful for visualizing how the subnet mask splits the address.
4Use the wildcard mask output directly in Cisco ACLs or OSPF configurations.
5Try the quick-fill examples (10.0.0.0/8, 172.16.0.0/12, etc.) to compare class A, B, and C subnets side by side.

主な機能

Instant CIDR calculation — network address, mask, broadcast, and host range appear as you type
Binary representation — see the IP and mask in dotted binary to understand bit-level subnet logic
Wildcard mask output — ready to paste into Cisco ACLs and routing configurations
Private/public detection — identifies RFC 1918 private address ranges automatically
Example quick-fill — common subnets (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/24) to explore immediately

よくある質問

詳しく読む

Subnet Calculator Guide — CIDR, Subnet Masks & IP Ranges Explained