example config for "automatic allocation"?

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Sep 29 07:50:43 UTC 2006

Chuck Kollars wrote:

>According to RFC 2131 there are three different
>possible methods of IP address allocation.
>One of them, "automatic allocation": assigning a
>permanent address to every client the first time, is
>exactly what I want.
>But I'm having an awful time configuring my DHCP
>daemon to actually do it.

That's because the ISC server doesn't support it, and neither does 
the Microsoft server. If you really do need it then you would have to 
build some additional software to detect new clients (to which you 
give temporary addresses with short leases) and then create host 
statements with fixed addresses. The next main release will support 
reserved leases, but you'll still need some external glue to make 

However, bear in mind that the ISC server also follows the bit in the 
RFC about trying to keep a clients address the same - even if it goes 
away for some time. This mechanism will effectively give you the same 

When a client 'goes away' and it's lease expires, the details are not 
deleted, but the lease is marked as available for re-use. If the 
client re-appears, possibly years later, it will be given the same 
address - exactly the functionality you are looking for.

However, while the client is away, there may well be other devices 
coming and going that need addresses - and sooner or later the server 
will need to re-use addresses. It does this by taking the least 
recently used free lease and reassigning it - and at this point, a 
returning client can no longer have the same address.

You are probably now thinking "but that isn't good enough", to which 
the response is :

If you permanently allocate addresses to clients, how are YOU going 
to do cleanup to free addresses so that new devices can join the 
network ? You HAVE to have a cleanup regime unless you have unlimited 
addresses to use, the only advantage you as a human admin have over 
the software is you can make decisions based on non-technical facts 
like "Oh, that system has been scrapped" (re-use the address) or 
"That user is on sabattical and will be back next year (don't re-use).

That only leaves the question - why do you NEED non-changing addresses ?

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