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Computers are getting IP address outside our DHCP range

Posted on 2013-01-02
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Last Modified: 2013-01-02
I have our DHCP address range set to 172.18.1.12 - 150.  However, two of the computers have IP address past this range.  Why would this happen?
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Question by:J.R. Sitman
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LVL 39

Expert Comment

by:Krzysztof Pytko
ID: 38736574
That looks like you have configured them with:

1) Static IP address
2) MAC address reservation is set up
3) The old DHCP server with the old scope is still active
4) IP address is 169.254.x.y/16 (APIPA) that means no connection to DHCP server

Can you verify that, please ?

Regards,
Krzysztof
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rharland2009 earned 1200 total points
ID: 38736576
If you run an 'ipconfig /all' on these computers' command lines, what is displayed as their DHCP server?
Are these computers addressed statically?
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Expert Comment

by:Perarduaadastra
ID: 38736584
That isn't a private IP address space, and so the addresses are routable.

I would suggest changing your internal LAN addressing to 172.16.x.x, which is a private address space. If the problem persists with the new addressing, you know that it is something on your LAN that's causing it.
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by:Thomas Grassi
ID: 38736585
What Ip addresses are they getting?

Are they getting a 169.254.x.x number?

If so they are not finding the DHCP server

If they have vailid ip addresses then they may be manually configured.

What OS are the computers?
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Author Comment

by:J.R. Sitman
ID: 38736736
When I run Ipconfig /all they both are pointing to a DHCP server that is incorrect.  one is getting 172.18.1.50 and the other is .51.  Our DHCP server is 172.18.1.8
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Assisted Solution

by:rharland2009
rharland2009 earned 1200 total points
ID: 38736748
Let's find out who the .50 and .51 devices are, since those devices are apparently handing out DHCP addresses.
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Expert Comment

by:Krzysztof Pytko
ID: 38736755
Try to use this simple and free tool to discover rogue DHCP servers
http://blogs.technet.com/b/teamdhcp/archive/2009/07/03/rogue-dhcp-server-detection.aspx

Krzysztof
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Expert Comment

by:Thomas Grassi
ID: 38736773
what happens when you ping 172.18.1.18  from these two computers?

on the dhcp server check what the mac addresses are for .50 and .51 and compare them to the computers ipconfig /all

post results
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Author Comment

by:J.R. Sitman
ID: 38736832
I can ping 172.18.1.8.  Also on the DHCP server 172.18.1.50/51 are in the excluded range.

When I ran the Rogue detection it finds our correct DHCP server as a Rogue server and it finds the 172.18.1.50/51
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Expert Comment

by:Krzysztof Pytko
ID: 38736843
Check if you have no DHCP running on any router device or if there is no DHCP emulation software installed. Just check MAC address of those devices and check the manufacturer

Krzysztof
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Author Comment

by:J.R. Sitman
ID: 38736847
I just typed 172.18.1.50 in IE and found out what it is.  It is our Wireless Adhoc device that is supposed to assign IP to wireless devices outside of our DHCP range.

So the question now becomes how do I get the two computers back on the proper DHCP server?
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Assisted Solution

by:Krzysztof Pytko
Krzysztof Pytko earned 200 total points
ID: 38736856
You can't :) Mostly (in 99%) the first DHCP reply is accepted by client. You need to implement VLANs and IPHelper to separate DHCP broadcast traffic

Krzysztof
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Assisted Solution

by:Thomas Grassi
Thomas Grassi earned 200 total points
ID: 38736861
One problem is you have more than one DHCP server on the same subnet. Not a good practice. If you do that you have to be very carefull of the ranges you are using.

What are the settings on your wireless router? comapre them to your dhcp server.

ipconfig /release then ipconfig /renew on the computers after you gt the dhcp ranges squared away.
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Author Comment

by:J.R. Sitman
ID: 38736879
Until I can get the company out here to reconfigure the wireless device, what problems can this create?
I'll post later, off to a meeting.  Thanks
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Expert Comment

by:DrDave242
ID: 38737044
If .50 and .51 are giving out different DHCP options (default gateway, DNS servers, etc.) from the ones being given out by the "real" DHCP server, machines receiving DHCP addresses from them could have trouble accessing other resources on the network.  If those options are the same, then you may not experience any issues at all unless, for example, you've got firewall rules restricting access based on IP ranges.

Also, a minor point of correction on an earlier respose:

That isn't a private IP address space, and so the addresses are routable.
172.16.0.0/12, which includes everything from 172.16.0.0 to 172.31.255.255, is private, so you're fine in that regard.
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Author Closing Comment

by:J.R. Sitman
ID: 38737285
Thanks to all for the help and advice.
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Expert Comment

by:Perarduaadastra
ID: 38737755
I stand corrected - I'd forgotten the /12...
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