Re: I am confused about DHCP

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2008/6/14 John Cornelius <jc@xxxxxxxxxxxxxxx>:
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

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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