Private/Public IP Calculator
Classify any IPv4 address as private, public, loopback, link-local, CGNAT, multicast or reserved.
Inputs
Address Scope
Private
Defining RFC
RFC 1918
Internet Routable
No
NAT Required to Reach Internet
Yes
Step by step
Values used
IPv4 address = 10.42.7.19
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.
Address Scope
= Private
Defining RFC
= RFC 1918
Internet Routable
= No
NAT Required to Reach Internet
= Yes
How it works
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
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.