What does /128 mean in IPv6?

Answer

A /128 matches exactly one IPv6 address: all 128 bits are fixed. It is the IPv6 equivalent of an IPv4 /32. ::1/128 is the IPv6 loopback address.

Worked example: 2001:db8::1/128. Type any block to compare.

2001:db8::1/128 is 1 address: 2001:db8::1 to 2001:db8::1.

20010db8000000000000000000000001

128-bit prefix 0 bits for subnets and hosts (each hex digit is 4 bits)

Network: 2001:db8::1
First address: 2001:db8::1
Last address: 2001:db8::1
Addresses: 1
Usable: 1
Expanded: 2001:0db8:0000:0000:0000:0000:0000:0001
Range type: Documentation · not routed on the internet

Special-purpose range

Documentation 2001:db8::/32 RFC 3849
Reserved for examples in documentation. Never routed.

Split it into subnets

Is this IP in the range?

Range to CIDR blocks

What to know

Use /128 where a rule or route should cover one address: a security group entry for a single server, a host route, or a router loopback address used for management and BGP sessions.

Gotchas

Allowlisting a client by /128 is fragile. Devices using privacy (temporary) addresses change their address regularly, often daily; allow the client's /64 instead, or the whole site prefix.
Do not assign a /128 to an interface that needs neighbors. Hosts on a LAN need the subnet's /64 prefix length, or they will not know their neighbors are on-link.

Related explainers

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