Apache version numbering is similar to what the linux kernel used before 2.6.<p>In a.b.c, odd b indicates the unstable or experimental branch, and even b&#x27;s are used for major and backwards-compatibility breaking changes.
Backwards-compatibility preserving changed can be introduced in c.<p>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.