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STRANGE Internet Connecitivty Issue In Network

Posted on 2014-04-21
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Last Modified: 2014-05-28
We have had a rather odd issue arise in our network over the past few days.  We had a Comcast modem (in bridge mode) that goes to a Cisco E1500 and multiple switches behined it (unmanaged simple 5 and 8 port switches).  

A few days ago this started... computers would get an IP and all the information (ran an ipconfig /all) and open one or two web pages then go dead--cant resolve host nor open web page--then connectivity comes back--then drops out for good.

We upgraded to a DLink DIR-655 and a DLink 16 port switch.  Still having the issue.  The DLink router is in default configruation and no other servers/routers on the network.

Where to start with the troubleshooting?  It is a network that was working fine until a coupe of days ago and nothing has changed--just a DHCP network with Windows PCs.

I have often seen devices that don't connect or won't pull IP address--but what would cause intermittent connectivity issues that will not go away.
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Question by:gta2011
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John Hurst earned 500 total points
ID: 40013852
What is providing DHCP?  The Comcast modem or the D-Link Router?  

If both, fix that as you only want the router to provide DHCP in this small network.

Are any computers set for static IP?  They should not be in most cases. Set them for DHCP.

Try a test:  Turn off the computers, turn off the router and disconnect the modem. Start the router, and turn on the computers one by one. None will have internet but ALL should have IP addresses. Do they?  Try this and let us know.
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by:Scott Thomson
ID: 40013891
As john says above.

Ipconfig a bunch of machines and see which one they think is the default gateway (the one providing the internet normally)

go into each of th routers by typing their ip address into the web browser normally http://10.0.0.1 or http://192.168.0.1

make sure only 1 thing is providing the DHCP addresses. if you have 2 then it will cause you some issues when the second dhcp goes to hand out ip addresses. especially if it hands out ip's on a different range.

post your ipconfig if possible and then give us the ip addresses of the devices and we can help further.

if you want you can post the config pages of the 2 devices.
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by:Rob Williams
ID: 40013950
When you loos connectivity can you access a web page using its IP such as Google  http://74.125.226.113/
This may help to isolate if it is a DNS or connectivity issue.
If you can't access by IP, run ipconfig, do you still have an IP and gateway?
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by:Scott Thomson
ID: 40013963
Also have you contacted your internet provider to have them run a trace on the line for you. see if they can pick up any noise etc..?
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by:tankergoblin
ID: 40013981
Did you try to put static ip?? if this happen with static ip?
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by:Rob Williams
ID: 40013982
If it is not a DNS issue you may also want to try a tool like Netgong:
http://netgong.tsarfin.com/

Netgong is basically a ping tool that will monitor multiple IP's and log failures.  It can be set to ping as many devices as you like as often as once per minute.  Therefore set it up to ping the router, the router's gateway (your ISP), and an internet IP.  When it fails you will be able to see where the connection is dropped; between PC and route, Router and ISP, or ISP and Internet.
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by:tankergoblin
ID: 40013984
try to manually set you dns
you can try 8.8.8.8 see things you mention happen??
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by:Bing CISM / CISSP
ID: 40018705
the symptom sounds like there are two DHCP servers staying on the same network (broadcast domain) and offering IP addresses for two subnets with only one having access to the internet.

i would GUESS, for somereasons, your Comcast modem's LAN port was mistakenly linked to Cisco E1500's LAN port (should be WAN port) or one of the downstream switches. even you have replaced wireless router and switches, but the incorrect connection was kept. probably because of too many cables there? ;-)
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by:Rob Williams
ID: 40019003
Though multiple DHCP servers can certainly cause problems, it would be unusual for a PC to randomly refresh their IP before the DHCP lease expired.
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by:John Hurst
ID: 40019112
@gta2011 - Can we hear back from you please about our suggestions?
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