What is a /128 in IPv6?

A /128 is a single IPv6 address — all 128 bits are fixed. It is used for host routes, loopback (::1/128), and pinning a service to one specific address.

What is the difference between /128 and /64?

A /64 is a full LAN subnet with 2⁶⁴ addresses and 64 host bits. A /128 fixes all 128 bits and identifies exactly one address — it is a host route, not a subnet you subdivide.

When should I use a /128 host route?

Use /128 for loopback (::1/128), anycast service endpoints, or static routes that must match one specific address in routing policy — not for numbering a VLAN or Wi-Fi network.

How do I calculate an IPv6 /128 subnet?

Enter an IPv6 address and select prefix /128 in the calculator above — or paste full CIDR notation like 2001:db8:abcd:0012::1/128. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2⁰ addresses.

IPv6 /128 Subnet Calculator

Results update as you type

A /128 identifies a single, specific IPv6 address — one host. It is the IPv6 equivalent of an IPv4 /32 host route. Loopback (::1/128) and individual host routes use this prefix.

Prefix length

/128

/128 = 2⁰ addresses (≈ 1)

Results for 2001:db8:abcd:12::1/128

Documentation (RFC 3849)Global scope

Network / prefixThe first address — identifies the subnet itself

First addressSubnet-router anycast; first address in the block

Last addressThe highest address in this block

Prefix maskEquivalent to /128

Total addresses≈ 1 addresses

Address typeGlobally routable scope

More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown

Neighboring /128 subnets

Expanded address
Compressed address
Network (expanded)
Last address (expanded)
Prefix mask
Total addresses (exact)
Reverse DNS (PTR)
Host bits / network bits

Hextet breakdown

20010db8abcd00120000000000000001

NetworkSplit groupHost

Quick facts for IPv6 /128

Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
Total addresses: 2⁰
Approx. count: 1
/64 subnets: —

IPv6 /128 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetnetnetnetnetnetnet

Network bits (128)Split hextetHost bits (0)

Network bits: 128
Host bits: 0
Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
Total addresses: 2⁰
Approx. count: 1
/64 subnets: —
Addresses formula: 2^0
/64 relationship: smaller than a /64

Overview

A /128 identifies a single, specific IPv6 address — one host. It is the IPv6 equivalent of an IPv4 /32 host route. Loopback (::1/128) and individual host routes use this prefix.

Common use cases

Single host routes
Loopback address (::1/128)
Anycast service endpoints

Key facts

A /128 fixes 128 network bits and leaves 0 host bits — 2⁰ total addresses.
In network design terms, /128 is typically a single-host route.
Written out, /128 holds exactly 1 addresses.

Design guidance

A /128 is a host route — one specific address. Use it for loopback (::1/128), anycast service endpoints, or static host routes in routing policy. Do not assign a /128 as a LAN prefix.

Practical example

2001:db8:abcd:0012::1/128 identifies one host. A routing table entry for 2001:db8:abcd:0012::1/128 pins traffic to that single address — useful for loopback, anycast, or host-specific policy routes.

Related RFCs and standards

RFC 4291IPv6 Addressing Architecture

Prefix sizing reference

/128 fits inside…

Supernet	Addresses	/128s inside
/127	2¹	2
/126	2²	4
/124	2⁴	16
/120	2⁸	256

Notable /128 networks

::1/128Loopback
::/128Unspecified address

Frequently asked questions

Common questions about IPv6 /128 blocks, prefix sizes, and use cases.

Common IPv6 prefix sizes

/3 /12 /16 /20 /24 /28 /32 /36 /40 /44 /48 /52 /56 /60 /64 /80 /96 /112 /120 /124 /126 /127 /128