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.
/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 <code>::1/128</code>, 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?
<code>::1/128</code>. 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