PCs dropping off the network randomly

By mskcool ·
Hi Guys,

Just have a bit of confusing situation. I have got a new client who has got issues on the network wherein the pcs are dropping randomly of the network. The setup is that they have got Cisco 2800 series router with Site to site VPN setup. They have one Win 2k3 DC server, 2 applications servers, one ISA server and one lotus notes server. In addition to this they have got a Zyxel modem which is used to connect to the internet.

Initially the ip address range was 10.135.12.xx on all the servers and the clients. They advised that recently they had to change the IP addresses on the servers and Clients. They have changed the IP address on the servers to 10.135.12.xx range with subnet of and using 10.135.13.xx range for the clients. The DHCP server is on the Win 2k3 server and the default gateway for the clients is that is of the cisco router ? I was advised that it forwards all the traffic to the ISA server and the traffic goes from there to outside world.

Since the IP address changes the pcs are getting disconnecting randomly from the network sometimes they come back on by themselves or you might need to disable and reenable the LAN connection in the pc. The pcs and servers are connected to 3com Switches. I have done the following troubleshooting:

assign static IP address to PC with 10.135.12.xx range as well as 10.135.13.xx range
rebooted the servers, cisco and the switches
disabled the firewall on the machines
when this happens cant ping to and from the affected machine.

Sorry for posting long question, any help and suggestions would be greatly appreciated.


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All Answers

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OSI Layer model & DHCP server

by netwrk_admn In reply to PCs dropping off the netw ...

DHCP Server, is it configured correctly? Anything unusual about lease times (try extending it to a number of days).

If you're familiar with the OSI layer, I would start going from bottom -> up. Find the affected PC, ensure all physical cables are connected, make sure the switch sees the MAC address, then ping the loopback IP (, then ping another computer on the subnet, etc.

You can also isolate the PC onto a hub with another PC using a packet sniffer to see what the **** the computer's doing, bonus if you can place the PC on a hub on the network and see the DHCP reply.

Hope this helps, it is a strange one you have one.

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I dunno I certainly wouldn't have used a Class A subnet

by CG IT In reply to PCs dropping off the netw ...

and DHCP doesn't always change PC addressing. In fact, once issued by DHCP, client addresses typically remain pretty much the same. DHCP just doesn't decide to "mix it up" for the **** of it and change everyone's address.

When you mention PCs dropping from the network, that could mean anything. Need more detail. Losing connectivity is dropping from the network. Losing internet access to many is "dropping from the network" but simply losing internet access doesn't mean they aren't connected to the network.

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re pcs dropping off the network

by mskcool In reply to I dunno I certainly woul ...

Thanks for your valuable suggestions

I have checked the DHCP server is configured correctly. There are about 30 pcs on the network and the issue happens randomly on random pcs. When the pc drops off the network I mean to say that its not able to access any network resources like shared drives, the lotus notes gives message that its disconnected from the server and the accounting package gives the similar disconnect message.

I have assigned static ip address on some pcs, when this issue occurs I have tried to ping the loop back address → success, ping the Static ip address that I have assigned to the pc ?> success, ie get replies, when I ping the gateway → says request timed out, ping a pc that is on the 10.135.12.xx range--> times out, ping IP address on the .13.xx range-> times out ping ISA →times out. When I disable and re enable LAN connection on the affected pc comes back up and I get replies from all the pings that I have mentioned above. I have also disabled symantec AV on the machine and tried but no joy. I have tried assigning both 10.135.12.xx ip and 10.135.13.xx ip -

I though that it might be issue with one of the switches ? but it happens on computers that are connected on either switch. I am not sure whether its issue with DHCP as it happens even when you assign the static IP address.

I might try connecting one of the pcs to a Hub and see ? but the glitch is that it happens randomly on differnet pcs.

One more thing that servers are assigned static ip address of the 10.135.12.xx range. Any help would be really appreciated.


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rule of thumb in troubleshooting, eliminate physical first

by CG IT In reply to re pcs dropping off the n ...

if you have 2 subnets, and the connectivity faults while trying to reach a different subnet, look to the router.

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if you suspect the switch

by netwrk_admn In reply to re pcs dropping off the n ...

It could be that the switch had been preconfigured for VLANs. Make sure they are all on VLAN 0 (default). Telnet or http to the switch if it supports it.

You're right with the DHCP - if it happens with static addresses, then we can rule out the DHCP for the time being.

The biggest thing I would concentrate on, is why can't the PC ping it's own default gateway - maybe the switch as you say. If it can't get there, then it can't reach the other subnet, of course.

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by mskcool In reply to if you suspect the switch

Sorry guys,

I am bit confused, not sure whether I have misunderstood the subnet concept. the servers are configured with 10.135.12.xx subnet address and the clients are on the 10.135.13.xx subnet - from what I understood was example computer with ip address with subnet is on the same subnet/network as the pc with the ip with subnet do they go through the router or just the switch? from my knowledge I think they communicate directly without the default gateway being involved. please correct me if I am wrong.

The switches are unmanaged switches therefore cannot telnet to them and cant run snmp -

I have removed the gateway setting from the one of the affected pcs - it works fine for sometime and then gets disconnected - the only way is I disable and re enable the LAN connection on that pc and then I can ping the 10.131.13.xx ip addresses as well as 10.135.12.xx ip addresses.

I will try few more things and let you know

Thanks a lot for your help and advise

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by CG IT In reply to subnets

10.135.12.XX/22 is not on the same subnet as 10.135.13.XX/22


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hah, didnt catch that

by netwrk_admn In reply to Subnets...

If you had a /23 subnet, it would if my math is correct. Otherwise, as CG IT stated, that subnet set up now is -, -, -, etc.

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IP is in the same Subnet.

by hardkash In reply to hah, didnt catch that

If i am not wrong both the IP's stated 10.135.12.x & 10.135.13.x come in the same network.

I did calculate it as shown below:

Sample Address: 00001010.10000111.000011 00.00001010

Netmask: = 22 11111111.11111111.111111 00.00000000

Default Subnet Mask: 11111111.00000000.000000 00.00000000

Wildcard: 00000000.00000000.000000 11.11111111

Slash Notation: 22

Network I 00001010.10000111.000011 00.00000000

Broadcast ID : 00001010.10000111.000011 11.11111111

HostMin: 00001010.10000111.000011 00.00000001

HostMax: 00001010.10000111.000011 11.11111110

Hosts/Net: 1022

So, if you have another IP address ranging 10.135.13.x it will be in the same network.


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AT LAAAAST issue is sorted

by mskcool In reply to PCs dropping off the netw ...

Thank you for your help. At last afetr much investigation found that the zyxel modem was sending out proxy arp messages and causing the issues - changed the modem and every thing works sweet now

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