The previous Apache HTTPD version was 2.2. The current one is 2.4. The next one will be 2.6. In their documentation these versions are sometimes called 2.1/2.2, 2.3/2.4 and 2.5/2.6 (in the titles of "New Features" pages). But I cannot find an explanation why there are numbers skipped, nor why they are sometimes sort-of bundled up in pairs...

Does anyone know?

CodePudding user response：

The odd numbers are for development, and the stable releases get the matching even number: 2.1, once stable, becomes 2.2.

This is described in the Apache release notes:

With the introduction of Apache 2.1, the Apache httpd project has adopted an odd-even release strategy, where development happens with alpha and beta releases assigned an odd-numbered minor version, and its general availability (stable) release is designed with the subsequent even-numbered minor version. E.g. 2.1.0-alpha through 2.1.6-alpha were followed by 2.1.7-beta through 2.1.9 beta, and cumulated in the 2.2.0 general availability release.