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IP Range CalculatorPRO
IPv4 Subnetting & RangeInstant Client-Side Tool

IPv4 /31 Link Calculator

Apply point-to-point addressing rules.

IPv4 /31 Link Calculator: inputs and results

Enter an IP address, CIDR prefix, or range. Then check its first and last address below.

Quick presets:

Calculated Boundaries & ParametersReady

Network Address192.168.1.0
Broadcast Address192.168.1.255
First Usable Host192.168.1.1
Last Usable Host192.168.1.254
Subnet Mask255.255.255.0
Wildcard Mask0.0.0.255
Total Addresses256
Usable Host Range254

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.
Open Full Interactive Suite

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Network Address Calculator

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IPv4 Broadcast Address Calculator

Find broadcast addresses where applicable.

First and Last Address Calculator

Identify a range’s endpoints.

Usable IPv4 Host Range Calculator

Apply the selected subnet-use rules.

How IPv4 /31 Link Calculator Works?

This tool works by splitting an address block into pairs of 2 addresses and assigning one address to each end of a point-to-point link, as defined in RFC 3021.

  1. Enter a parent block

    Type a block reserved for links, such as 10.255.0.0/28.

  2. Divide into /31 pairs

    Each /31 holds 2 addresses. A /28 (16 addresses) yields 8 links.

  3. Assign both ends

    The even address (.0) goes to side A and the odd address (.1) goes to side B.

  4. Compare with /30

    The calculator shows the addresses saved against /30 links, which use 4 addresses per link.

IPv4 /31 Link Calculator Formula

The formula is Links = 2^(31 − p) for a parent prefix p, with Side A = Network and Side B = Network + 1.

Links per parent

Links = 2^(31 − p)

A /24 parent yields 2^7 = 128 links.

Side A address

A = N (even)

The lowest bit is 0.

Side B address

B = N + 1 (odd)

The lowest bit is 1.

Addresses saved vs /30

Saved = 2 × Links

Each /31 uses 2 addresses, and each /30 uses 4.

Worked example: 100 router links

/30 design = 100 × 4 = 400 addresses (a /23 is needed)
/31 design = 100 × 2 = 200 addresses (a /24 holds them)
Saved      = 200 addresses (50 %)
Link 1     = 10.255.0.0/31 → A 10.255.0.0, B 10.255.0.1

IPv4 /31 Link Calculator Conversion Table

The table splits 10.255.0.0/28 into 8 point-to-point links, with the side A and side B address of each link.

Link/31 PrefixSide ASide BSame Link on /30
Link 110.255.0.0/3110.255.0.010.255.0.110.255.0.0/30
Link 210.255.0.2/3110.255.0.210.255.0.310.255.0.4/30
Link 310.255.0.4/3110.255.0.410.255.0.510.255.0.8/30
Link 410.255.0.6/3110.255.0.610.255.0.710.255.0.12/30
Link 510.255.0.8/3110.255.0.810.255.0.910.255.0.16/30
Link 610.255.0.10/3110.255.0.1010.255.0.1110.255.0.20/30
Link 710.255.0.12/3110.255.0.1210.255.0.1310.255.0.24/30
Link 810.255.0.14/3110.255.0.1410.255.0.1510.255.0.28/30

The /30 column shows where each link would sit in a /30 design. The /30 plan needs a /27 (32 addresses) for the same 8 links.

What is IPv4 /31 Link Calculator?

The IPv4 /31 Link Calculator is a tool that plans point-to-point links with 31-bit prefixes, where each link uses exactly 2 addresses and both are usable.

RFC 3021, published in December 2000, allows /31 prefixes on point-to-point links. A link between 2 routers needs no broadcast address, so the network and broadcast reservation is dropped. Cisco IOS, Juniper Junos, Arista EOS, and Linux all support /31 links.

A /31 design uses half the address space of a /30 design. Service provider backbones and data center leaf-spine fabrics with hundreds of links save entire /24 blocks this way.

What are the uses of IPv4 /31 Link Calculator?

There are 5 main uses of the IPv4 /31 Link Calculator, including backbone links, data center fabrics, and address conservation.

  • Numbering backbone links

    Internet Service Provider (ISP) cores number every router-to-router link with a /31.

  • Building leaf-spine fabrics

    A fabric with 4 spines and 32 leaves has 128 links. /31 numbering needs 256 addresses, which fits one /24.

  • Conserving public IPv4 space

    Public IPv4 addresses are scarce. /31 links cut link address use by 50 %.

  • Configuring BGP peerings

    Border Gateway Protocol (BGP) sessions between 2 routers often run over a /31 transfer network.

  • Documenting link addressing

    The A/B table gives a ready list of link addresses for configuration templates.