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CIDR Subnet Calculator

Calculate network boundaries, netmasks, broadcast addresses, wildcard masks, and usable host ranges in real time.

Presets: • •

What Is CIDR Notation?

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and IP routing. It replaces the old Class A/B/C system with a flexible prefix length (such as /24) indicating how many leading bits of the 32-bit IPv4 address belong to the network prefix.

1. Subnet Prefix Length

Specifies the number of network bits. For example, /24 provides 256 total IP addresses (254 usable hosts).

2. Network vs Broadcast

The first address in a subnet is reserved for the network identifier, and the last address is reserved for the broadcast address.

3. Wildcard Masking

Inverted netmask (e.g. 0.0.0.255) commonly configured in Cisco ACLs and OSPF routing protocols.

Frequently Asked Questions

A /24 subnet contains 256 total IP addresses. Subtracting the network address (first IP) and the broadcast address (last IP) leaves 254 usable host IP addresses.

The subnet mask for a /16 prefix is 255.255.0.0, which yields 65,536 total IP addresses and 65,534 usable host addresses.

In standard IPv4 networking, the lowest address in a subnet identifies the network wire itself, while the highest address is reserved to broadcast packets to all hosts on that subnet.

RFC 3021 specifies /31 subnets for point-to-point links between two routers, where both IP addresses are usable with no network or broadcast address required.

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