I am confused about DHCP

Antonio M antonio.montagnani at gmail.com
Mon Jun 16 13:33:50 UTC 2008


2008/6/14 John Cornelius <jc at hangarpilot.net>:

> Antonio M wrote:
>
>> I have a small LAN with DHCP running on Fedora 9. I checked the active
>> leases: this is the list of active leases
>> 1)I don't understand how IP address is connected to MAC address, i.e. if a
>> MAC has already an IP address, should a new lease be started with same MAC
>> address?? I see 5 IP's connected to same MAC address
>> 2) How are  the IP adressess released?? I would expect 192.168.0.63 <
>> http://192.168.0.63> after 192.168.0.62 <http://192.168.0.62> and so
>> on... (please note that 00:16:d4:dc:a7:08 sometimes is started by F9 and
>> sometimes by F10
>>
>>
>> 200 IP addresses available, 6 allocated (3 %)
>>
>> IP Address           Ethernet                         Hostname       Start
>> Date                  End Date
>> 192.168.0.62 <http://192.168.0.62>        00:16:d4:dc:a7:08        acer
>>              2008/06/11 07:07:52  2008/06/18 07:07:52
>> 192.168.0.224 <http://192.168.0.224>      00:16:d4:dc:a7:08
>>              2008/06/12 06:39:18  2008/06/19 06:39:18
>> 192.168.0.155 <http://192.168.0.155>      00:1a:80:23:e3:7b
>> PC-contecsrl     2008/06/12 07:03:23  2008/06/19 07:03:23
>> 192.168.0.158 <http://192.168.0.158>      00:16:d4:dc:a7:08
>>               2008/06/13 07:47:01  2008/06/20 06:50:47
>> 192.168.0.241 <http://192.168.0.241>      00:16:d4:dc:a7:08
>>               2008/06/13 07:22:33   2008/06/20 07:22:33
>> 192.168.0.90 <http://192.168.0.90>        00:16:d4:dc:a7:08
>>                2008/06/13 07:47:01   2008/06/20 07:47:01
>> 192.168.0.155 <http://192.168.0.155>      00:1a:80:23:e3:7b
>> PC-contecsrl     2008/06/13 07:50:57 2008/06/20 07:50:57
>>
>> --
>> Antonio Montagnani
>> Skype : antoniomontag
>>
>>
> The problem arises because you seem to have an impractically long lease
> time. Note that the lease issued on 2008/06/11 doesn't expire until
> 2008/06/18 so your lease time seems to be 7 days. Most DHCP servers only
> issue leases for 24 hours and those are renewed by the client after
> (typically) 12 hours.
>
> Each of the IP addresses for your Acer probably represents a reboot of the
> Acer. When the Acer boots it makes a DHCP request and the server makes
> several checks to see if an address is in use and one of those checks is its
> own database. If the address has already been assigned it selects another
> and makes the test again.
>
> Apparently F9's DHCP server doesn't check to see if a MAC address is
> already associated with an IP address lease that is unexpired so the best
> thing to do is to shorten the lease time to something more practical, like
> 86400 seconds (one day). That will clean out the database when the leases
> expire.
>
> I have no idea why addresses are not assigned sequentially but usually
> nobody cares. Perhaps the person coding it was having a bad day and wanted
> to do something perverse. You can actually get better control over it by
> reducing the size of the address space to a small multiple of the number of
> machines on your network. From the DHCP listing it looks like there are only
> 2 machines being serviced by DHCP so a reasonable number might be 16 or 32
> addresses instead of 200.
>
> John Cornelius
>
>
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>

Tnx John and others

I reduced lease time to 24 hr.
I have still some comments:

In the lease list I note today that only Fedora computers are repeated while
Windows Vista is appearing only once!!! and I am pretty sure that it was
connected to the network every morning. What is different between a Fedora
computer and a Windows computer??? Or am I missing something???

-- 
Antonio Montagnani
Skype : antoniomontag
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