What is an IP address? IPv4, IPv6, public and private addresses explained

02IPv4 vs IPv6: what is the difference?

IPv4 addresses are 32 bits long, which allows 4,294,967,296 addresses (2^32), written as four decimal numbers from 0 to 255. IPv6 addresses are 128 bits long, written as eight groups of four hexadecimal digits; RFC 5952 drops leading zeros and shortens the longest run of zero groups to ::, so 2001:0db8:0000:0000:0000:0000:0000:0007 becomes 2001:db8::7.

They are separate protocols, so an IPv4-only host cannot talk to an IPv6-only host without a translator, and most networks run both side by side (dual stack). IPv6 has now passed a milestone: on 28 March 2026, 50.10% of users reaching Google did so over native IPv6, the first time the share passed half, as the Internet Society's Pulse reported.

03Public vs private IP addresses

A public address is globally unique and routed on the internet; a private address is valid only inside one network and is never routed between networks. RFC 1918 (February 1996) reserved three IPv4 blocks for private use so that every home and office can reuse them, and a router performing network address translation (NAT) rewrites outgoing packets to its single public address.

Two blocks are often mistaken for private space. 100.64.0.0/10 is shared address space for carrier-grade NAT (RFC 6598): if your router's WAN side shows an address from it, your ISP shares public addresses between many customers, and connections started from the internet cannot reach you directly. The documentation ranges from RFC 5737 and RFC 3849 exist so that examples, like the ones on this page, never point at a real host.

Loopback, link-local, IPv6 unique local and the other special-purpose blocks, with the ones an edge should filter, are listed in private IP address ranges.

04How do I find my IP address?

There are two answers, and they usually differ. Your public address is the one websites see: the IP and ASN lookup shows it, with the network and prefix it belongs to. Your local address is the one your device holds on the home or office network, usually from a private range, and the operating system shows it.

If the two differ, a router is translating addresses, which is normal. If the lookup places you in another country, you are probably behind a VPN, a corporate proxy or a mobile carrier's gateway, and the address belongs to that exit.

# Linux: every interface and its addresses, one line each
ip -brief address show

# Linux: the source address and gateway used for an internet destination (sends nothing)
ip route get 192.0.2.1

# Linux: global IPv6 addresses only
ip -6 address show scope global

# Windows: addresses, subnet masks, gateways and DNS servers
ipconfig /all

05Who assigns IP addresses?

Addresses are handed down a chain. IANA, whose functions are performed by PTI under ICANN, allocates large pools to the five regional internet registries: AFRINIC, APNIC, ARIN, LACNIC and the RIPE NCC. They allocate to members, mostly ISPs, hosting companies and large organisations, which assign addresses to customers. A device then gets one from the network it joins, by DHCP on IPv4 or, on IPv6, usually by building its own from the prefix the router advertises.

Holding addresses is separate from routing them. A network announces its blocks with BGP under its autonomous system number, and the registries record who holds each block, which is how lookup tools find the operator behind an address.

The dates that shaped today's address supply:

• September 1981: RFC 791 defines IPv4 and its 32-bit address
• February 1996: RFC 1918 reserves the private ranges
• December 1998: IPv6 is specified in RFC 2460, revised as RFC 8200 in July 2017
• 3 February 2011: IANA's free IPv4 pool runs out and its last five /8 blocks go one to each registry
• April 2012: RFC 6598 sets aside 100.64.0.0/10 for carrier-grade NAT
• 25 November 2019: the RIPE NCC makes its last regular IPv4 allocation and moves to a waiting list
• 28 March 2026: IPv6 passes 50% of users reaching Google for the first time

06What does an IP address reveal about you?

Less than most people fear. An address reliably identifies its network and usually the country, but not a person or a street address. MaxMind, a large geolocation provider, puts country-level accuracy above 99%, state or province level at 55% to 80% and city level typically at 20% to 75%, depending on the country and the type of connection.

Sharing is the other limit. Behind a home router every device shows the same public IPv4 address, and behind carrier-grade NAT or a VPN thousands of unrelated users can. IPv6 makes addresses more individual, which is why operating systems create temporary, rotating addresses for outgoing connections (RFC 8981).

07How IP addresses are grouped into networks

Every address has two parts: a network prefix shared by all hosts on the same network, and a host part unique within it. The number after the slash in 192.168.1.10/24 says how many leading bits are network, and a subnet mask writes the same boundary in dotted form. Routers on the internet only look at prefixes, which is why the smallest block that travels reliably in BGP, the /24, matters to anyone who wants their own addresses routed.

08Why IP addresses matter in DDoS attacks

Network floods are aimed at an address, not at a domain name. Once attackers know a server's public IP, they can send traffic straight to it and bypass anything placed in front of the website, which is why sites behind a proxy keep the origin address private: see how to hide your origin IP.

Source addresses are a weaker clue than they look. In direct UDP floods the source is often forged, and in reflection attacks the packets come from innocent third-party servers, so blocking either can hit an innocent party. In a botnet-driven DDoS attack the real sources number in the thousands, many of them home connections behind shared addresses. Filtering therefore has to look at traffic patterns, not only at address lists.