DHCP "static" assignments

Simon Hobson simon at thehobsons.co.uk
Thu Aug 8 07:08:53 UTC 2013

Gregory Sloop wrote:
>So, if there's no pool, but a defined host, the DHCP server will hand
>out that IP for any host with the matching MAC address? Really!??!

Yes, that is the point of a host declaration with a fixed-address statement.

>I was under the impression there had to be a pool for the DHCP server
>to hand addresses out of

Only for dynamic leases. That's what the pool is, a pool of addresses for dynamic leasing.

Steven Carr wrote:
>IMHO this is a much simpler model than defining all available space
>and then restricting it afterwards, just define what you want to
>actually use and ignore the rest.

The MS stance also has historical baggage. Way back in NT4 days (the only time I've fiddled with MS DHCP), once you defined a scope it was not possible to edit it - ie if you said that the scope allowed (say) .100 to .199 to be used for dynamic leases, then that's what you were stuck with for all time - unless you deleted the scope and re-created it (which of course means re-setting all options one at a time via the GUI). Thus it became standard practice to include the entire subnet and exclude the addresses you don't want it to use.
I don't know whether that restriction has changed, but old habits die hard !

Personally I think the MS way sucks (as does the lack of capability in their software) - but I can see that for someone "brought up on it" it will seem natural.

And lastly, sorry lost who wrote that you can have the fixed-address within a pool and it's OK. No you cannot, it is NOT OK.
Any fixed addresses must **NOT** be in any dynamic range<period>. Certain safety features may (or these days, more likely may not) help you, but if you overlap the config like this then there is a good chance that eventually the server will attempt to hand out the same address to two clients.
The main thing that will stop this is the "ping before lease" feature. IFF the client with the fixed address is online AND it responds to pings AND you haven't disabled the ping before lease function AND this is an initial offer rather than a renewal, then the server will detect this (it gets a ping reply from that address) and mark the dynamic lease as abandoned. Abandoned leases are only use in the last resort when all other leases are in use - so it would possibly appear to the casual observer that "there's no problem".
Remove any of those conditions and you risk getting a duplicate address allocation on your network - which is considered "not good".

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