IP Subnet Calculator: CIDR, Subnet Mask, Wildcard Mask, Host Range

Understanding IP Subnets

Subnetting divides an address space into predictable networks. This calculator translates CIDR notation, subnet masks, wildcard masks, and planning requirements into network address, broadcast address, usable host range, address counts, and split subnet tables.

What is CIDR?

CIDR notation writes a network as IP/prefix, such as 192.168.1.10/24 for IPv4 or 2001:db8::/48 for IPv6. The prefix length tells you how many bits belong to the network portion. A larger prefix (like /28) means a smaller subnet with fewer available addresses, while a smaller prefix (like /16) means a larger subnet.

IPv4 vs IPv6 at a glance

• IPv4: 32-bit addresses like 203.0.113.25. Includes a broadcast address and a count of usable hosts. IPv4 space is limited, so subnetting and NAT are common.
• IPv6: 128-bit addresses like 2001:db8::1. No broadcast (uses multicast), and the address space is enormous, making hierarchical planning easier.

Formulas

• IPv4 total addresses = 2^(32 - prefix).
• Usual IPv4 usable hosts = 2^(32 - prefix) - 2.
• IPv6 total addresses = 2^(128 - prefix).
• Wildcard mask = inverse subnet mask.
• Network address = IP bitwise AND subnet mask.

Worked /26 example

For 192.168.1.77/26, the subnet mask is 255.255.255.192. The block size is 64, so the /26 blocks in the last octet start at 0, 64, 128, and 192. Address 77 falls in the 192.168.1.64/26 block, broadcast is 192.168.1.127, and the usual usable range is 192.168.1.65 to 192.168.1.126.

How usable hosts are calculated (IPv4)

In IPv4, most subnets reserve the first address as the network address and the last as the broadcast address. That is why usable hosts are usually total - 2. Two special cases are common in real deployments and appear in results:

• /31: point-to-point, both addresses usable.
• /32: single host.

How to use this calculator

1. Enter an IPv4 or IPv6 address and its prefix length.
2. Choose the appropriate version if prompted.
3. Review the calculated network, broadcast (IPv4), and usable range.
4. Use the total hosts and mask to size your subnet.
5. Adjust the prefix to compare larger or smaller networks.

Choosing a prefix length

• Smaller number (e.g., /16) → larger network, useful for many devices or growth.
• Larger number (e.g., /28) → smaller network, good for tight segmentation.
• IPv6 best practice: LANs use /64; sites often receive /48 or /56 for clean hierarchy.

Complete Reference Tables

IPv4

*Special cases: /31 point-to-point (2 usable), /32 single host.

IPv6

FAQ

What is a subnet mask?

A subnet mask marks which IPv4 bits are network bits and which bits are host bits. For example, 255.255.255.0 is the dotted mask for /24.

What is CIDR notation?

CIDR writes a network as IP/prefix, such as 192.168.1.10/24. The prefix length is the number of network bits.

How do I calculate usable hosts?

For normal IPv4 subnets, calculate 2^(32 - prefix) total addresses, then subtract 2 for the network and broadcast addresses.

Why does /31 have two usable addresses?

RFC3021 allows both addresses in an IPv4 /31 to be used on point-to-point links, so the calculator reports two usable addresses.

What is a wildcard mask?

A wildcard mask is the inverse of an IPv4 subnet mask. It is common in ACLs because zeros mean fixed bits and ones mean bits that may vary.

What subnet do I need for N hosts?

Use the reverse mode. It finds the smallest prefix where usable hosts are at least the number you enter, including the normal network and broadcast reservations.

What is the difference between IPv4 and IPv6 subnetting?

IPv4 has 32-bit addresses and broadcast addresses. IPv6 has 128-bit addresses, no broadcast, and commonly uses /64 for LANs.

Does IPv6 have broadcast?

No. IPv6 uses multicast instead of broadcast.