Attempting to find unused IP addresses on a network

Taking a server/networking class starting in January, but I need to know a trick or two right now, if anyone's willing to teach.

I have 20-some computers I need to configure for use on our network. I can configure 8 at a time. My favorite option would be to find 8 consecutive IP addresses on the network that are usable. To do that, I use the ping command in the command line.

However, I learned something the other day: Just because the ping returns 4x "request timed out" messages doesn't mean the IP address is free, it just means that IP isn't being used right that second. (Made the mistake of installing a computer to that IP, and we lost a printer - had to go back and re-do it.) I'd like to avoid that this time.

Is there a command that will let me find "ping request could not find host" - which I understand to be a message meaning that IP isn't being used AT ALL, instead of just RIGHT NOW - for a series of IP addresses? Or do I have to ping all 255 addresses on the network in order to find those?

 

Simply put no, you can ping any system on my LAN, they won't respond (the firewalls won't let them). Configure your network from the router, not individual desktops... The router knows what's going on.

 

Have a look at Spiceworks. It's a free network management utility that can auto-discover most of a network. The help system will also describe how to get all your systems detected.

Using this utility, you'll not only be able to find out quickly how your IP address space is being used, but also a lot more useful information. It's a very handy tool to have installed. (Its also uses the Rails framework so I like it even more )

As it runs continuously in the background, it will keep track of PCs that have temporarily been switched off.

Also, this problem highlights that it is a good to use DHCP. It means that for the majority of systems you don't have worry about the IP addresses they use. The DHCP server will do the work for you. However, don't allocate all your IPs to DHCP. Leave some free for static assignment (by limiting the range of IPs used, or adding exclusion ranges) where that is needed (e.g. servers and routers).

 

He subnets to make sure we have plenty of IP addresses for anything he'd like to assign. For instance, looking at the last two octects tells me which floor of the building and which computer I'm looking at: xxx.xxx.111.111 is the Lake Woebegone floor, and that's Garrison's computer, but xxx.xxx.222.222 is is the New York floor and Kieth's computer.

And these are replacements, anyway. As soon as I have them installed with everything, we'll just replace the boxes and use the same IP addresses. To follow the above example: I have a new Gateway that I set up on xxx.xxx.111.999, and when we take out Garrison's old eMachine at .111, then the Gateway becomes 111.111 and the eMachine gets wiped for storage, repurposing or sale.

So, I'm fortunate enough to not be trying to set up a network around this kind of scheme. I just needed some temporary static IPs to connect to the network for setup.

Thanks for the tip on Spiceworks. I'll check it out. I also found the neat toy that is Angry IP Scanner while I was googling.

 

Hmmm. You've not convinced me that's actually useful

I was digging around and found a couple of batch files I wrote to do this sort of thing a few years ago:

pingtest.bat

Code:

rem This utility is used with pingthem.bat to detect whether a node 
rem has responded to a ping A log file pingtest.log is created which lists 
rem the results. 

Echo --- Test started - %date% %time% --- > pingtest.log
for /L %%A in (1,1,254) do pingthem.bat %%A
Echo --- Test completed at %time% --- >> pingtest.log

pingthem.bat

Code:

@echo off
rem this batch file is used with pingtest.bat to create individual entries 
rem in to a ping test log. it uses a temporary log (ping.tmp) to store the
rem results of a ping and then tests it for success the results are then 
rem sent to the pingtest.log

ping 192.168.0.%1 -a -n 1 > ping.tmp
find  "Received = 1" ping.tmp && Goto Success

:Failure
echo 192.168.0.%1 failed >> pingtest.log
Goto EOF

:Success
echo 192.168.0.%1 !!!!passed!!!! >> pingtest.log

:EOF

However, to be honest its more an example of how far you can push bat files than a very useful application. It's nice in that it create an output log, but its very slow and doesn't get over personal firewall and switched off PC problems.

It would be tempting to write a little Ruby utility to do it, but why when there are utilities like Spiceworks that do it already.

 

"Practice?"

 

Sounds like a good place for DHCP reservations by MAC address.
And a spread sheet data base of all the info.