IPv4 uses 32 bits per address. That gives about 4.3 billion possibilities, which looked generous in 1981 and has since run out: RIPE NCC, the registry for Europe, allocated its last free IPv4 blocks in November 2019.
What they look like
- IPv4: four numbers from 0 to 255, separated by dots — 145.100.1.1
- IPv6: eight groups of hexadecimal characters, separated by colons — 2001:610:188:410:145:100:1:1
Why IPv6 is so much larger
IPv6 uses 128 bits. That is not four times as many addresses but 2⁹⁶ times as many — a number with 39 digits. Every household is given an entire /64 block as standard, which by itself holds more addresses than the whole IPv4 internet.
What you have
Many Dutch lines have both at once: dual-stack. Your device then chooses per connection, and under RFC 6724 usually prefers IPv6. The top of the home page shows which of the two this connection is using.
Why IPv4 is not going away
Too much still runs on it. Providers keep IPv4 alive with techniques like carrier-grade NAT, where hundreds of customers share one public address. That is exactly why your public address sometimes appears to be someone else's: it is also someone else's.