How is this different from IPv6 Expand/Compress?

Expand/Compress works with a single address, converting its form. This calculator works with an entire subnet: given an address and prefix, it computes the range boundaries (first and last address) and the total address count.

What prefix do ISPs typically assign?

Home users typically get /56 or /64 — both provide a huge address pool. /64 is the standard size for a single physical subnet (required for SLAAC autoconfiguration), while /56 lets you split the block into 256 /64 subnets for different VLANs.

Why does an IPv6 subnet have so many addresses?

IPv6's 128-bit address space is deliberately huge — even a standard /64 subnet contains 2^64 (about 18 quintillion) addresses. This removes the need for NAT: every device can have its own public address, and the huge pool simplifies autoconfiguration and route aggregation.

IPv6 Subnet Calculator Online

IPv6 Subnet Calculator

Range boundaries and address count for an IPv6 subnet by CIDR

IPv6 Subnet Calculator: compute a subnet's range

Unlike IPv4, where address space was a limiting factor requiring careful subnet mask selection, IPv6 uses a fixed 128-bit address space with an enormous surplus. The calculator accepts an address in CIDR notation (address/prefix, e.g. 2001:db8::/32) and computes the range boundaries: the first address (network address) and the last address of the subnet, plus the exact address count.

How IPv6 masking works

The prefix specifies how many of the 128 leading bits belong to the network identifier; the remaining bits are available for host numbering within the subnet. The network address is obtained by zeroing all host bits (AND with the mask); the last address by setting all host bits to one (OR with the inverted mask). The number of addresses in a subnet always equals 2 raised to (128 − prefix).

Typical IPv6 subnet sizes

/48 — the standard block an ISP assigns to an organization; allows 65536 /64 subnets
/56 — a typical home internet block; gives 256 /64 subnets for VLAN separation (main network, guest, IoT)
/64 — the standard size for a single physical subnet, required for SLAAC autoconfiguration; holds 2^64 addresses
/127 — used for point-to-point links between routers (the IPv6 equivalent of a /30 in IPv4)
/128 — a single host, one address with no range

Why IPv6 doesn't need NAT

Even the minimally recommended /64 subnet holds around 18 quintillion addresses — enough to give every device on a network its own public IPv6 address without address translation (NAT), which in IPv4 was a forced workaround for address scarcity. This simplifies end-to-end connectivity and removes some of the classic NAT headaches (P2P connections, VoIP, protocol design).

Frequently asked questions

How is this different from IPv6 Expand/Compress?
Expand/Compress works with a single address, converting its form. This calculator works with an entire subnet — computing range boundaries and address count from address and prefix.
What prefix do ISPs assign?
Typically /56 or /64 for home users. /64 is standard for one physical subnet (needed for SLAAC), /56 lets you split into 256 /64 subnets.
Why so many addresses per subnet?
The 128-bit space is deliberately huge — even /64 holds 2^64 addresses. This removes the need for NAT — every device can have its own public address.