Private/Public IP Calculator
Classify any IPv4 address as private, public, loopback, link-local, CGNAT, multicast or reserved.
About Private/Public IP Calculator
Classification is a range test on the first two octets. Anything that falls outside every reserved block is globally routable space assigned by a regional internet registry. Knowing whether an address is RFC 1918, CGNAT or public determines whether a flow needs NAT, whether it can be filtered at the edge, and whether it may legally appear in the global routing table.
Formula
Private/Public IP
The address is matched against the reserved ranges 10/8, 172.16/12, 192.168/16, 127/8, 169.254/16, 100.64/10, 224/4 and 240/4.
- o1
- First octet
- o2
- Second octet
Worked example
With the default values below, Private/Public IP Calculator returns Address Scope of Private. Change any input above and every figure here is recalculated in your browser.
Inputs used
- IPv4 address
- 10.42.7.19
Results
- Address Scope
- Private
- Defining RFC
- RFC 1918
- Internet Routable
- No
- NAT Required to Reach Internet
- Yes
Step by step
- 1Values used
- 2Private/Public IP
- 3Address Scope= Private
- 4Defining RFC= RFC 1918
- 5Internet Routable= No
- 6NAT Required to Reach Internet= Yes
Assumptions and limits
- The result is derived only from the single input 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.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Private/Public IP calculated?
The address is matched against the reserved ranges 10/8, 172.16/12, 192.168/16, 127/8, 169.254/16, 100.64/10, 224/4 and 240/4. Classification is a range test on the first two octets. Anything that falls outside every reserved block is globally routable space assigned by a regional internet registry.
Why does Private/Public IP matter?
Knowing whether an address is RFC 1918, CGNAT or public determines whether a flow needs NAT, whether it can be filtered at the edge, and whether it may legally appear in the global routing table.
What values do I need to enter?
This calculator takes 1 input: IPv4 address. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.