What is an IP address? IPv4, IPv6 and private networks
An IP address identifies an interface on an IP network so packets can be delivered to a destination. It is a network address, not a permanent identity for a person or even a particular device. A computer can have several addresses at once, and an address can change when the connection changes.
IPv4 and IPv6 addresses
IPv4 uses 32 bits and is normally written as four decimal numbers, for example 192.0.2.1. IPv6 uses 128 bits and hexadecimal groups, for example 2001:db8::1. These examples come from documentation ranges; they are not addresses to assign to a public server. Both protocols may be active on the same device.
A network interface may have multiple IPv6 addresses with different scopes and lifetimes. Some addresses are assigned manually, while a router, DHCP service or Internet provider supplies others. The address visible in the operating system is therefore not necessarily the address a website sees.
Public, private and shared addresses
A public address can be routed on the Internet, but a firewall or router may still prevent inbound connections. Private IPv4 ranges include 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are used within networks rather than routed directly across the public Internet.
Home routers often translate private addresses with NAT. Providers may also use carrier-grade NAT, where multiple customers share a public IPv4 address. Consequently, knowing a public IP alone does not identify one household, account or device. Compare the router’s WAN address with an external lookup when diagnosing this situation.
What an IP lookup can tell you
Registration and routing records can suggest the network operator and approximate service area. Geolocation databases can be wrong, particularly for mobile connections, VPNs, corporate gateways and recently reassigned blocks. An IP address does not reliably reveal someone’s name, street address or exact physical position.
If a connection fails, first check whether the device has an address and default route. Then compare local connectivity, DNS resolution and access to the intended service. Record the time of a test when sharing findings with an operator, because addresses and routes can change.