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Loopback Addressing Plan Calculator

Work out loopback addressing plan instantly with clear inputs, formula shown and shareable results.

About Loopback Addressing Plan Calculator

Loopbacks are /32 host addresses that stay up regardless of physical interface state, so they are used as router IDs, BGP peering sources, management targets and MPLS LSP endpoints. Reserving one contiguous block — commonly out of 10.255.0.0/16 — lets every loopback be covered by one summary route and one management ACL.

Formula

Loopback block

addresses = devices x loopbacks per device; covering prefix = 32 - ceil(log2(addresses)); each loopback is configured as /32

Worked example

With the default values below, Loopback Addressing Plan Calculator returns Loopback addresses needed of 120. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of devicesminimum 1
60
First loopback address
10.255.0.1
Loopbacks per deviceaccepts 1–8
2

Results

Loopback addresses needed
120
Last address in the range
10.255.0.120
Covering block prefix
25 /n
Spare addresses in the block
8

Step by step

  1. 1Values used
  2. 2Loopback block
  3. 3Loopback addresses needed= 120
  4. 4Last address in the range= 10.255.0.120
  5. 5Covering block prefix= 25 /n
  6. 6Spare addresses in the block= 8

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why configure loopbacks as /32?

A /32 wastes nothing and cannot be affected by link failure, so protocols that source packets from it stay stable even when physical interfaces flap.

Should loopbacks come from a dedicated block?

Yes. A dedicated range keeps summarisation tight and makes management-plane filters, SNMP allow-lists and TACACS rules simple to write.

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