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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