A private IP is an address used only inside a local network. It is not routable on the internet: public routers drop packets destined for those ranges.
Reserved ranges (RFC 1918)
10.0.0.0/8— large corporate networks.172.16.0.0/12— from 172.16.0.0 to 172.31.255.255.192.168.0.0/16— the home network default.
127.0.0.0/8 (loopback) and 169.254.0.0/16 (link-local) are also reserved.
How a private IP becomes public
Your router uses NAT: it rewrites the packet's source address with your public IP before sending it out. The reply returns to the router, which delivers it to the right device using its translation table.
Why it matters
If a tool shows a 192.168.x.x address as your public IP, something is wrong with the lookup. The IP Checker always displays the address seen by external services, which is the public one. Diagnostic tools also refuse queries against private ranges to prevent misuse against internal networks.