Subnet Calculator: IPv4 & IPv6, CIDR, netmasks

Calculate network address, broadcast, netmask, wildcard, host range and reverse DNS for IPv4 and IPv6 subnets. Entirely in the browser, no server roundtrip, no tracking.

Input

IPv4 IPv6

Quick examples: 1.1.1.1 2606:4700:4700::1111 192.168.1.0 100.64.0.1

Result

Subnetting in 60 seconds

An IP address has two parts: the network portion (which network?) and the host portion (which device on the network?). The prefix length decides where the split happens; e.g. /24 means the first 24 bits are network, the last 8 are host.

IPv4 has 32 bits, i.e. 2³² ≈ 4.3 billion addresses worldwide.
IPv6 has 128 bits, i.e. 2¹²⁸ addresses, more than grains of sand on Earth.
In IPv4, /24 is a typical LAN, /16 a larger corporate network.
In IPv6, /64 is the smallest sensible subnet size (for SLAAC), /48 a typical end-customer allocation.
The netmask is a binary mask with 1s for the network portion. /24 gives 255.255.255.0.
The wildcard mask is the bitwise inverse, used in Cisco ACLs.

What this calculator computes

Whether you're planning a private IPv4 range or splitting an IPv6 allocation:

Network address and broadcast (IPv4), or first/last address (IPv6)
Netmask in decimal, wildcard mask, prefix length
Host range with first and last usable address, plus number of possible hosts
Class (A through E) and type (PRIVATE, LOOPBACK, LINK-LOCAL, MULTICAST, RESERVED, CGNAT, GLOBAL-UNICAST)
Integer, hex and binary IDs for scripting and database storage
in-addr.arpa and ip6.arpa reverse DNS notation
RFC 1924 base 85 short notation for IPv6
6to4 prefix and IPv4-mapped IPv6 address
Microsoft IPv6 literal (UNC path compatible)

Frequently Asked Questions

What is CIDR and what do I need it for?

CIDR (Classless Inter-Domain Routing) is the notation for IP ranges with variable prefix length, e.g. 10.0.0.0/24. The number after the slash specifies how many leading bits belong to the network address. /24 means 256 addresses, /16 means 65,536. Since the 90s, CIDR has replaced the rigid class system (A/B/C) and allows finer-grained subnet sizes.

Why does /24 only have 254 usable hosts and not 256?

Within an IPv4 subnet the first address (network address) and the last address (broadcast) are reserved. So /24 has 256 total addresses, of which 254 are usable for hosts. IPv6 has no reserved broadcast address; every address in the range is usable.

What's the difference between /31 and /30?

A /30 has 4 addresses (network + 2 hosts + broadcast). A /31 has only 2 addresses, both usable (RFC 3021); this is commonly used for point-to-point links between routers because no broadcast is needed there. /32 is a single host address, often used on loopback interfaces.

How many /64 subnets fit into a /48?

2^(64-48) = 65,536 subnets. That's also why providers typically delegate IPv6 with /48 or /56 to end customers: every device gets its own /64.

What's the difference between private and public IP addresses?

Private IPv4 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routable on the public internet and are used for internal networks. In IPv6, Unique Local Addresses (fc00::/7) play the same role. The calculator detects and labels the address type for you.

What does the "wildcard mask" mean?

The wildcard mask is the bitwise inverse of the netmask. It's mainly used in Cisco ACLs (Access Control Lists) to describe IP ranges. Example: netmask 255.255.255.0 corresponds to wildcard 0.0.0.255.

What is the ip6.arpa entry?

It's the reverse DNS notation for IPv6, analogous to in-addr.arpa for IPv4. You need it when setting up PTR records for your IPv6 addresses (e.g. for clean mail delivery). The calculator generates it automatically.

Are my IP inputs stored?

No. The subnet calculator runs entirely in your browser via JavaScript. No server communication takes place; your inputs never leave your machine.

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