Addresses

IPv4 vs IPv6

IPv4 and IPv6 are two ways of numbering a machine on the internet. You can have one, the other, or both. The box above says which one carried this page. This note is the rest of the distinction: the format, why both still exist, and what changes for you when a network is IPv4-only, IPv6-only, or dual-stack.

The numbers

IPv4 is 32 bits, written as four decimal bytes from 0 to 255, separated by dots. 203.0.113.8 is an IPv4 address (that particular one is reserved for documentation). The whole space is about 4.3 billion addresses, and large slices are reserved for private use, multicast, and other special purposes. What is left was not enough once phones, home routers, and servers all needed public addresses, which is why the price of IPv4 went up and why ISPs started sharing.

IPv6 is 128 bits, written in hexadecimal with colons. Runs of zeros collapse to :: once in an address. 2001:db8::1 is the documentation shape. The space is large enough that the usual design gives a household a prefix, often a /56 or a /64, instead of a single shared host address. You will not memorize an IPv6 address. You also should not have to. Copy it from this page when a form asks for it.

What this connection is using

A browser on a dual-stack network usually prefers IPv6 when the path works, and falls back to IPv4 when it does not. This site sees only the family that arrived. The other line on the homepage is a second attempt, not a second reading of the same socket. If that attempt fails, or the name answers on the wrong family, the line says the address was not reached. It does not keep a loading state. Forcing a family from a terminal is curl -4 https://ipinfo.fyi or curl -6 https://ipinfo.fyi.

Why networks still run IPv4

A lot of software, and a lot of corporate networks, still assume a dotted IPv4 address. Some home equipment does not route IPv6 to the LAN even when the ISP provides it on the WAN. Some services have no IPv6 at all, so a v6-only client reaches them only through a translator the ISP operates. Turning IPv6 off to “simplify” a home network is a local choice. It does not free a public IPv4. The ISP is still short of those, which is why CGNAT and private addresses are doing so much of the work.

What IPv6 changes in practice

On a line with working IPv6 you often have a real prefix of your own and a shared IPv4. Deutsche Telekom’s DS-Lite, some Japanese MAP-E and DS-Lite deployments, and Indian mobile networks are all versions of that pattern: IPv6 is the unshared path, IPv4 is the scarce one. Inbound services that speak IPv6 can be reached on the prefix, subject to the router’s firewall. Services that speak only IPv4 still see the shared address.

Privacy extensions replace the host half of a temporary IPv6 address on a schedule. A website that logged yesterday’s full address may see a different host half today even though you did not change ISP. Firewalls and allow-lists should be written against the prefix you were assigned, not against a single full address you copied on a Tuesday, unless you have turned privacy extensions off for that host on purpose.

Which one to give someone

Give them the address this site shows for the family their software speaks. If they said “IP” with no version, they almost always mean the public IPv4, and they will reject an IPv6 string. If the IPv4 line is a shared CGNAT address, giving it to them will not let them connect inbound. Say so, and use IPv6 or a tunnel instead. The per-system steps show where each operating system prints the private address, so you do not send someone 192.168.1.23 and wonder why it fails from their house.