Solved: Changed ISP and now some network computers not working

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bobothesmart

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Jan 20, 2011
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Good Morning,

I am a teacher at a K-8 School and I have also been roped into being the go-to Tech Guy as well. Our building used to have a very old server that basically just served as a place for teachers to store files and not much else. We have just changed ISPs and decided that we no longer needed the server at all. I disconnected the server from the network and replaced the old ISP's modem with the new ISP's modem. At first, everything seemed OK. My computer and several other teachers' connected to the Internet with no problem. However, some of the computers in the building could not connect. We get error messages with "limited or no connectivity."
Part of me thought that perhaps the connections themselves are bad. However, when I take my laptop to classrooms with trouble connecting - mine connects easily using their cables. If I move their computers to my room, their laptops still won't connect.
I have put my computer side-by-side with another one to make sure the settings were the same (auto-detect IP, DNS, etc.) and can't find differences. This problems is affecting our Windows 7, Vista, and XP computers the same.

I would appreciate any thoughts about what to try next.

Thanks in advance!
 
Joined
Jan 7, 2011
Messages
8
For everyone having an issue getting onto the internet, refresh their dchp lease.

winxp
start -> run -> cmd -> ipconfig /release then ipconfig /renew

win7
start -> cmd -> ipconfig /release then ipconfig /renew

If its as plain jane as you say it is, they are holding dchp leases from either that server you unplugged, or the last isp's router/modem. And schools, like any other corporate environment have users that keep their machines on 24/7, which in this scenario, keeps their lease active. This old lease has an entry for the old isp modem (gateway) and not the new one.

hope this helps.
 

bobothesmart

Thread Starter
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Jan 20, 2011
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I just tried that on a Win7 machine that wasn't working. When I did the ipconfig /release, I got this message:
An error occured while releasing interface Local Area Connection: An address has not yet been associated with the network endpoint.

When I tried ipconfig /renew:
An error occured while renewing interface Local Area Connection: The DHCP client has obtained an IP address that is already in use on the network. The local interface will be disabled until the DHCP client can obtain a new address.

Any thoughts?
 
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It is exactally what it says it is.

That machine has obtained an IP from the dhcp server that is already being used on the network.
This could be due to some other machine on your network having a static IP assigned thats in the
address range that the DCHP is assigning from. Heres an example.

Computer1 (I need a IP please!!)
DCHP -> Computer1, my pool is 10.10.10.1 to 10.10.10.254, you may have 10.10.10.3
Computer1 configs itself with 10.10.10.3

Unfortunately, Computer2 has already been assigned to use 10.10.10.3 by some user.

This seems like a network admin issue. You really shouldnt have static ip's addressed in the same
pool that the dhcp is leasing from unless you register that static ip with the dhcp so that it doesnt
try and lease it out.

To quick remedy this, and mind you this is a bandaid fix, You can set a static ip on Computer1 somewhere higher in the ip pool range. So if your IP was 10.10.10.3, and its already taken, set a static to 10.10.10.50 and see if you get connectivity to the gateway. If you set it static, you will need to also
fill out the information about its subnet, gateway and dns server. You can find this on your laptop when you do ipconfig.

*** This is assuming a very simple network topology of 1 router, 1 gateway, 1 dchp, ect ect ect. this issue may be much more complex, and impossible to troubleshoot unless a network map can be made.
 
Joined
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Also, since there are multiple computers with old leases.. It may not be a static issue at all.. You may need to just find every computer that does not have connectivity and ipconfig /release all of them.. then one by one renew their dhcp leases.
 

TerryNet

Terry
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You mentioned no router. Are you really just using a modem and switch and getting multiple public IP addresses from the ISP? If so, how many, and does that correspond with the number of computers that can successfully connect?

It may help a lot if you give us the brand and model of your modem and any routers and switches and how they are interconnected.
 

bobothesmart

Thread Starter
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Jan 20, 2011
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I will look up the model numbers when I get a chance to check them out.

I don't have a firewall setup for the network yet. If I were to get that hooked up (we have a SonicWall) and connect that between the modem and the hubs/switches, is it possible that the firewall could take care of the DHCP end and properly distribute IPs and fix the problem? (I apologize if this makes no sense - I'm definitely out of my league here with some of this networking...).
 

bobothesmart

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Jan 20, 2011
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We are using Verizon Business DSL, so the we are using Veriznon's modem/router. I can login to the router via IE or Firefox. Is the router taking care of DHCP? If so, is there a way I can make sure that it is assigning IPs properly?
 

TerryNet

Terry
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Probably in the LAN section you can check how many addresses are in the Dhcp server's address range. Commonly this is about 100, but may be 200+ or just a few. Obviously you want to make sure that it is not too few.

There also is usually at least a Dhcp client list showing all the IP addresses currently assigned. Sometimes there are even better lists, showing all connected devices.
 

bobothesmart

Thread Starter
Joined
Jan 20, 2011
Messages
10
I want to thank everyone for their help. I finally figured it out - and the solution was simple. (It's embarrassing that I didn't figure it out sooner). Our router with Verizon was only set up to handle a range of 32 IPs, which was insufficient for the buidling. I increased the number and now we are having no problems.

Thanks again!
 

TerryNet

Terry
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You're welcome. :)

Just curious--was the restriction in the Dhcp server's address range or in the Subnet Mask?
 
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