address range ... not on net

Benjamin Wiechman benw at meltel.com
Wed Jul 2 20:21:24 UTC 2008


> -----Original Message-----
> From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On
> Behalf Of Simon Hobson
> Sent: Wednesday, July 02, 2008 1:10 PM
> To: dhcp-users at isc.org
> Subject: RE: address range ... not on net
> 
> Benjamin Wiechman wrote:
> 
> >In the case of a shared network as described, how does the dhcp server
> >determine which subnet to use when assigning an address? If I would have
> two
> >subnets would it default to one until filled, then assign addresses from
> the
> >second or would I need to use classes or some other mechanism to assign
> >clients to one subnet or the other?
> 
> It is 'undefined' - ie in a shared network, unless you tell the
> server otherwise then it assumes all addresses are equal and it can
> give any available address to any client. In practice there is an
> order due to the (undocumented and liable to change without notice)
> internal mechanism the server uses, this being to allocate addresses
> 'top down'.
> 
> If you wish to have certain clients in a particular subnet then you
> must tell the server how to do it with appropriate allow and deny
> clauses in the pools. The criteria can be something basic like
> 'known' or 'unknown', but is more usually done via classes.
> 

No, the undocumented behavior is fine. What we really want is simply a
larger pool of addresses. I just wanted to clarify that when subnet A has no
more free leases that the server will revert to subnet B and continue to
serve addresses, or serve some mix of addresses in an arbitrary fashion
until all available leases are handed out from all available subnets in the
shared network.

We have nodes in our wireless network that are exceeding a /24 network, but
we can't justify adding a second /24 to the site (ARIN would hang us...). I
didn't realize you could use a shared network on a network with dhcprelay.
All the discussion I had seen pointed to using it to serve two networks
connected to the dhcp box. 

This would be a very simple solution for us to add a second, smaller IP
block to the site without difficult to manage/document VLAN configurations,
etc. 

Ben Wiechman
Network Admin
Wisper High Speed Internet





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