I am confused about DHCP
John Cornelius
jc at hangarpilot.net
Sat Jun 14 17:57:35 UTC 2008
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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