IPv6 /127 Link Planner
Divide a parent prefix into address pairs.
IPv6 /127 Link Planner: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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How IPv6 /127 Link Planner Works?
The IPv6 /127 Link Planner works by dividing a parent prefix into /127 blocks of 2 addresses, one block per point-to-point link, as recommended by RFC 6164.
Enter a parent prefix
Type a prefix reserved for links, such as
2001:db8:ffff:100::/120.Count the links
A /120 holds 256 addresses, which gives 2^(127 − 120) = 128 links.
Assign both ends
Each /127 has an even address for side A and an odd address for side B.
Print the link table
The planner lists every link with its prefix and both endpoint addresses.
IPv6 /127 Link Planner Formula
The IPv6 /127 Link Planner formula is Links = 2^(127 − p), Side A = Parent + 2k, and Side B = Parent + 2k + 1.
Links per parent
L = 2^(127 − p)p is the parent prefix length.
Kth link prefix
N_k = Parent + 2kk starts at 0.
Side A / Side B
A = N_k, B = N_k + 1Last hex digit even / odd.
Reserved /64 practice
1 × /64 reserved, 1 × /127 configuredRFC 6164 suggests reserving the /64 for each link.
Worked example: 2001:db8:ffff:100::/120
Links = 2^(127 − 120) = 128 Link 1 = 2001:db8:ffff:100::/127 → A ::0 B ::1 Link 2 = 2001:db8:ffff:100::2/127 → A ::2 B ::3 Link 8 = 2001:db8:ffff:100::e/127 → A ::e B ::f
IPv6 /127 Link Planner Conversion Table
The IPv6 /127 Link Planner conversion table lists the first 8 of 128 point-to-point links carved from 2001:db8:ffff:100::/120.
| Link | /127 Prefix | Side A | Side B |
|---|---|---|---|
| Link 1 | 2001:db8:ffff:100::/127 | 2001:db8:ffff:100:: | 2001:db8:ffff:100::1 |
| Link 2 | 2001:db8:ffff:100::2/127 | 2001:db8:ffff:100::2 | 2001:db8:ffff:100::3 |
| Link 3 | 2001:db8:ffff:100::4/127 | 2001:db8:ffff:100::4 | 2001:db8:ffff:100::5 |
| Link 4 | 2001:db8:ffff:100::6/127 | 2001:db8:ffff:100::6 | 2001:db8:ffff:100::7 |
| Link 5 | 2001:db8:ffff:100::8/127 | 2001:db8:ffff:100::8 | 2001:db8:ffff:100::9 |
| Link 6 | 2001:db8:ffff:100::a/127 | 2001:db8:ffff:100::a | 2001:db8:ffff:100::b |
| Link 7 | 2001:db8:ffff:100::c/127 | 2001:db8:ffff:100::c | 2001:db8:ffff:100::d |
| Link 8 | 2001:db8:ffff:100::e/127 | 2001:db8:ffff:100::e | 2001:db8:ffff:100::f |
What is IPv6 /127 Link Planner?
The IPv6 /127 Link Planner is a tool that numbers IPv6 point-to-point links with /127 prefixes, giving each link exactly 2 addresses.
RFC 6164, published in 2011, recommends /127 prefixes on inter-router links. A /64 on a point-to-point link leaves the link open to Neighbor Discovery exhaustion attacks, where a sender scans millions of unused addresses and fills the neighbor cache. A /127 leaves no unused addresses to scan.
Many operators reserve a full /64 for each link in the plan and configure only the /127 on the interfaces. The planner supports both styles.
What are the uses of IPv6 /127 Link Planner?
There are 4 main uses of the IPv6 /127 Link Planner, including backbone numbering, fabric design, and security hardening.
Numbering backbone links
Service provider cores assign a /127 to every router-to-router link.
Designing data center fabrics
Leaf-spine fabrics with hundreds of links need a clean, sequential /127 table.
Hardening against ND exhaustion
Links with /127 prefixes leave no space for Neighbor Discovery cache attacks.
Generating configuration templates
The A and B columns feed directly into interface configuration templates.