dhcpd version relationship

[Berg, M. van den]

Hello,
 
Now available are versions 3.0.6, 3.1.0 and 4.0.0a2
Can anyone explain what the relation between those versions is.
For example: 
- if you have 3.0.6 on two servers in failover mode can one be replaced with 3.1.0?
- is 3.1.0 the successor to 3.0.6?
- what is the difference between 4.0.0 and 3.1.0? both are feature releases.
- etc, etc
 
 
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Meindert van den Berg