IPv6 is an Internet Protocol version with a much larger address space than IPv4. Addresses use hexadecimal groups separated by colons. A device or service may support IPv4, IPv6 or both, so two IP-checking tools can show different address formats without either being broken.
A practical example
An address such as 2001:db8::10 is a documentation example. The double colon is a shorthand for a sequence of zero groups. It is not a sign that part of the address is missing from the connection.
What to check
In a support report, keep the address family with the result: say that the tool reported an IPv6 address, for example. If you are comparing VPN behavior, use the same endpoint before and after connecting. Record any difference separately from whether ordinary browsing works. Do not disable IPv6 simply because its appearance is unfamiliar.
What it does not tell you
Seeing an IPv6 address is not automatically a leak. You need to know the expected routing and what the particular test measures. Likewise, a successful IPv4 check cannot establish the behavior of every IPv6 connection on a device.
Read the technical reference for the underlying definition. For a connection that is failing now, use the troubleshooting guide. To choose an app, follow the VPN selection worksheet.