# IPv6 /128 Prefix – Single-Host Routes Explained

> An IPv6 /128 prefix contains 1 addresses. Mask math, examples, typical roles and FAQs for planning IPv6 networks.

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# IPv6 /128 Prefix ReferenceIPv6

A single address, pinned with surgical precision: the /128 is IPv6's host route. Loopback interfaces carry them, routing tables pin them for steering traffic, and firewall rules target exactly one machine.

An IPv6 /128 prefix contains 1 addresses. A /128 identifies exactly one interface.

## /128 facts at a glance

- **Total addresses:** 1 (20)
- **/64 subnets inside:** —
- **Host/interface bits:** 0
- **Example network:** 2001:db8::/128
- **First address:** 2001:db8::
- **Last address:** 2001:db8::
- **Typical role:** Single host, loopback-style route

## A /128 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /128 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/128 | 2001:db8:: | 2001:db8:: |

## How an IPv6 /128 prefix works

A /128 prefix pins the first 128 bits of a 128-bit address, leaving 0 bits free. That free portion holds 20 = 1 distinct addresses, and every such block begins at a multiple of that size. Prefixes longer than /64 cannot use SLAAC; plan DHCPv6 or static assignment instead.

## Where /128 is used

- Router loopbacks, including ::1/128, the local loopback address.
- Host routes for blackholing, anycast steering, or maintenance pinning.
- Precise firewall and ACL entries that match one node and nothing else.

RFC 4291 designates ::1/128 as the loopback address, IPv6's counterpart of 127.0.0.1.

When writing addresses in text, follow the canonical form from RFC 5952: lowercase hexadecimals, leading zeros suppressed, and the longest zero run compressed exactly once.

## Frequently asked questions

**What does /128 mean in IPv6?**

All 128 bits are fixed, so the prefix identifies exactly one address, the same way a /32 pins a lone IPv4 host.

**What is the IPv6 equivalent of 127.0.0.1?**

::1/128. Traffic sent there loops back within the local node per RFC 4291.

**How many addresses are in a /128?**

One. There is nothing to subtract and nothing else can match.

**When do routes use a /128?**

For host routes: blackholing malicious sources, anchoring BGP sessions to loopbacks, or steering traffic to one specific device during maintenance.

## Open this prefix in the calculator

[Open 2001:db8::/128 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F128)

### Related IPv6 prefixes

[/29](https://itinsighthub.com/ipv6/29/)[/32](https://itinsighthub.com/ipv6/32/)[/34](https://itinsighthub.com/ipv6/34/)[/36](https://itinsighthub.com/ipv6/36/)[/40](https://itinsighthub.com/ipv6/40/)[/42](https://itinsighthub.com/ipv6/42/)[/44](https://itinsighthub.com/ipv6/44/)[/47](https://itinsighthub.com/ipv6/47/)[/48](https://itinsighthub.com/ipv6/48/)[/52](https://itinsighthub.com/ipv6/52/)[/56](https://itinsighthub.com/ipv6/56/)[/60](https://itinsighthub.com/ipv6/60/)[/64](https://itinsighthub.com/ipv6/64/)[/127](https://itinsighthub.com/ipv6/127/)

← IPv6 /127 prefix

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