Mapping a MAC to an IP...

Simon Hobson dhcp1 at
Wed Jan 21 17:12:52 UTC 2009

Peter Laws wrote:

>>Host declarations are global in scope, and so are valid even if a 
>>client is connected to a different subnet (in which case it will 
>>get a dynamic address from that subnet rather than the fixed 
>>address). But what would happen is that the client would get a 
>>suitable IP address, but inherit options (such as routers) from the 
>>subnet where the host statement is declared - I think you can 
>>imagine the confusion that causes !
>That makes no sense ...  if a particular host connects to a 
>different subnet, I'd want it to get the information *appropriate 
>for that subnet*, not the one where it may have a static 
>IP/reservation/whatever you want to call it.   What am I missing?

It's only a problem if you declare a host statement inside a subnet 
declaration - I think it also applies with class declarations as 
well. As long as you put your host statements in the global scope (ie 
not inside a subnet declaration) then they will work as you would 

It's a side effect of the way inheritance works, and probably 
something not envisaged when the original design was done. If you put 
a host statement inside a subnet, it inherits options (eg routers) 
from that subnet, but since it is a global object it is still 
recognised when the client is in a different subnet - and hence you 
find a client getting options that are inherited from a different 

The safe thing to do is to keep all your host (and class) 
declarations in the global scope - then you don't need to worry about 

Simon Hobson

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