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Old 04-03-2008, 06:42 AM   #1
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Finding Free Ip In Lan


Am administering LAN in my office, if some body asks for a free IP address, am searching my DHCP file and giving....

Is there any easier way of doing this.

I can ensure that, all the systems will be assigned IP by my DHCP server.

Is there any tool to find free IP addresses in my LAN ???

Any thoughts ?
Old 04-03-2008, 07:00 AM   #2
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Why do you have to search for free IP addresses?Can't you assigned them once for all?How big is your LAN?When using DHCP,you should have more then enough IP addresses within network range to choose which one to assigne to every computer on the LAN.
Old 04-03-2008, 07:27 AM   #3
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Do you mean that you are assigning static addresses outside the DHCP assigned range of IPs?

I would recommend tracking static IP addresses in a spread sheet.
Old 04-03-2008, 08:52 AM   #4
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you can always use an easy script to see the adresses that are already in use in your subnet. Ofcourse it won't show the adress of the machine from where you run it but that's a minor inconvenience :-)

So let's supose your subnet is then write the next script with your favorite editor:

for i in `seq 1 254`
ping -c 1 -w 1 192.168.2.$i 2>&1 &
arp 192.168.2.$i
done | grep ether

Save it, give it a name, make it excecuteable and you can run it anytime you need to.
I hope this will do the thrick for you ;-)


Last edited by lobo23000; 04-03-2008 at 09:04 AM.
Old 04-03-2008, 09:53 AM   #5
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Messing with your network can be tricky, so you should first make certain which fixed ip's you already gave out, and what the scope for your dhcp server is ( you should actually have some control over the scope of your dhcp server if you are admin ). When the dhcp range is defined and/or limited, you can be sure that the rest can be for fixed ip's ( for specific servers/services).

Then afterwards, there is ( or should be ) less need to do a discovery, and you could even try to implement some more security.

You can configure your dhcpd to also administer the fixed ip's. In your dhcpd.conf edit or make a section for fixed ip-addresses :

# Put fixed IP addresses here
host whatsinaname {
  hardware ethernet 00:28:50:29:DA:19;
For additional security access lists on routers/proxies will definitely restrict unauthorized usage, and if you are really making an effort RADIUS and/or other access control could be used, but that is the next step.

But always document your changes!

Last edited by grumbl; 04-03-2008 at 10:16 AM. Reason: put in dhcpd example, adding security advice ...


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