Windows blocks pings: dup IPs being handed out

Simon Hobson dhcp1 at
Thu Feb 26 19:36:31 UTC 2009

Colin McInnes wrote:

>Under default Windows firewalls, pings are blocked. So when the lease
>pool is all used up, and a request comes in, a ping is sent out to see
>if an address is still in use. But the Windows boxes don't respond,
>because they block pings. So an address is handed out under the
>assumption that the unit using it is offline, but it's already in use,
>just not properly responding.
>What alternatives to verifying addresses via ping are reasonable
>solutions for the server to check if an IP is still in use (short of
>making lease times much shorter)?

Your network is seriously broken - not responding to pings does NOT 
result in such behaviour on it's own (it's really just a 'safety 

If the Windows box got it's address via DHCP, then there will be an 
active lease and the server will NOT offer the address to any other 
device. If the Windows box did NOT get it's address from the DHCP 
server, then how on earth did it get an address it isn't entitled to 
use ?

You must NOT have ANY devices using address that are in a DHCP pool 
that aren't getting their address by DHCP from that pool - and that 
includes manually configuring the device, or giving it a 
fixed-address declaration in your DHCP config.

Simon Hobson

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