[dtech]

Apache version numbering is similar to what the linux kernel used before 2.6.

In a.b.c, odd b indicates the unstable or experimental branch, and even b's are used for major and backwards-compatibility breaking changes. Backwards-compatibility preserving changed can be introduced in c.

So 2.2 and 2.4 are pretty different versions, while 2.3 and 2.4 are closer except 2.3 is unstable. 2.4.0 is theoretically compatible with 2.4.17, except the latter has a lot more functionality and features.