What Is an IP Address? Public vs Private, IPv4 vs IPv6

Short answer

An IP address is a unique numerical identifier assigned to every device connected to a network. Every packet of data on the internet is addressed with a source and destination IP, like postal addresses on letters. Without IP addresses, the internet would have no way to route anything.

IPv4 vs IPv6

• IPv4 — 32 bits, written as four numbers 0-255 separated by dots: 172.67.133.42. Only ~4.3 billion possible addresses; we've long since run out.
• IPv6 — 128 bits, written as eight groups of hex separated by colons: 2606:4700:10::ac43:852a. ~340 undecillion addresses; enough for every atom on Earth.

Full comparison: IPv4 vs IPv6.

Public vs private addresses

Certain IP ranges are reserved as "private" — they only work inside a local network (your home, office, or data center) and can't reach the public internet:

• 10.0.0.0 – 10.255.255.255 — large private range, often used by corporate networks
• 172.16.0.0 – 172.31.255.255 — medium private range
• 192.168.0.0 – 192.168.255.255 — the one in your home router by default
• 127.0.0.0/8 — loopback (127.0.0.1 = this device itself)
• 169.254.x.x — link-local (when DHCP failed)

Your laptop's IP is almost certainly private. Your router has a public IP assigned by your ISP, and it uses NAT (Network Address Translation) to share that one public IP among all your devices.

Static vs dynamic

• Dynamic — assigned by DHCP when you connect. Can change. Default for home internet, hotel Wi-Fi, most consumer services.
• Static — permanently assigned. Required for anything you need to reach at the same address (servers, VPN endpoints, security cameras reachable from outside).

What an IP reveals

Looking up a public IP usually tells you:

• Owner — the ISP or hosting provider it's assigned to (ARIN/RIPE database)
• Approximate location — country and city (geolocation databases), usually accurate within 50-100 km; sometimes much less
• ASN — Autonomous System Number identifying the network
• Reverse DNS — if set, maps the IP back to a hostname
• Reputation — known spam source, Tor exit, VPN endpoint, data center

Check any public IP with IP address lookup.

Special addresses to know

Address	Meaning
127.0.0.1	Localhost — this device
::1	IPv6 localhost
0.0.0.0	"All interfaces" when binding a server
255.255.255.255	Broadcast to entire local network
8.8.8.8	Google Public DNS
1.1.1.1	Cloudflare Public DNS

CIDR notation — specifying ranges

192.168.1.0/24 means "the IPs 192.168.1.0 through 192.168.1.255." The number after the slash is how many leading bits are fixed. /24 means 24 fixed bits, leaving 8 bits (256 addresses). /16 is much larger (65,536 addresses). Subnetting calculations use binary — convert with IP to binary.

Related tools

Resolve a domain to its IP with DNS lookup. Get owner/location/reputation for any IP with IP address lookup.