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diagnose: on a network with a dozen computers, two computers interfere with each other

Z

ZippyDan

Platinum Member
ive got two computers that cant seem to access the network at the same time!

evidence:

turn on chat program on one computer, the other computer can't surf the internet. since chat program is constantly accessing network, other computer seems to be 'blocked out.' it can contact website and download a bit, but it will hang forever. turn off chat program on other computer, and suddenly surfing at blazing speeds. additionally, i can reverse this experiment and it works vice versa computers

play a LAN game. when both computers are logged in, everything is choppy and delayed by many seconds, as if computers are interfering with each other. log one of the two computers out and everything is fine.

this is in a room with 4 computers on an 8 port switch. that 8 port switch is daisy chained to the rest of the house which has as many as 8 other computers online at any time. ive tried switching ports on the 8 port switch to no avail. moreover, ive had this exact same setup for almost a year with no problem, it only started today when i reinstalled OS on one of the two computers.

i checked comp's ipconfig and they both have differnt IPs. and on that note, IPs are assigned by DHCP from linksys router in basement...

help

~Zippy!
 
Do you have Comcast cable who just bought your previous ISP's network?

Very odd that it would only affect two computers, if it was caused by a reinstall on one of them.
 
i have Earthlink DSL

yes, and i forgot to mention, the other two computers on same switch, are not affected an do not affect anything

~Zippy!
 
Dunno really. I just asked about Comcast because we lost the ability to have two machines gaming when Comcast took over. Getting DSL fixed that. 🙂

I suppose you could packet sniff each machine and see what traffic is coming in and going out. Obviously they're hitting each other with lots of packets. If they were flooding out the switch's port to your Net connection, it would make all your computers have a problem getting online. Win2k and XP come with a sniffer that allows you to see the traffic on the one machine it's installed on. I forget what it is though.

You could also install a network monitor just to see whether they're really pushing traffic or if it's something like a DoS that uses low traffic levels but can bring down a machine. I use NetStat Live from http://www.analogx.com and I quite like it, it's simple.
 
You could try setting both NICs to 10megabit / half duplex and go from there. One or both of them could be running at full duplex when they don't have that capability and flooding the node with collisions. If nothing else, it's something to look at...

Speed
 
thats what i thought too. but ipconfig /all reports different IPs. maybe its lying to me...

both computers are k7s5a boards with built in ethernet, and ive been running them for almost 2 years with no problems. and they are certainly capable of full duplex

~Zippy!
 
i double checked in network connections, one drive is on 101 and another is on 102

however. the 101 computer doesnt show up in my linksys router's DHCP table at all...

...of course, i think ive had this problem of computers randomly not showing up in my linksys DHCP table for as long as i can remember, and everything always worked fine...

~Zippy!
 
Do those same two computers interfere with each other, no matter where you place them in the network? Or, is it a situation where any two computers connected to that specific network node interfere with each other? Something, somewhere seems to think that those two computers are one and the same. Is there a particular LOGIN server on your network, that all PCs have access to? Could both systems be seen as occupying the same "space" as a LOGIN on that server? I think the two computers need to be isolated from each other in a manner that keeps them from seeing each other entirely. That might solve the problem, by placing them in separate user groups with a firewall between perhaps. Otherwise, I'd consider a complete reinstall of the operating system on at least one of the machines. Wipe it clean, start from scratch, and see what happens from there.

HUN-YA!

Red Ronin
 
Originally posted by: ZippyDan
i double checked in network connections, one drive is on 101 and another is on 102

however. the 101 computer doesnt show up in my linksys router's DHCP table at all...

...of course, i think ive had this problem of computers randomly not showing up in my linksys DHCP table for as long as i can remember, and everything always worked fine...

~Zippy!
Click to expand...

Upgrade your routers firmware.

 
Hmmm...just to be certain, look at ipconfig again and see whether both machines happen to have the same MAC address...it'd be odd, but it could happen. The MAC address is writable on that board and is often lost with certain BIOS updates.
 
Heres an idea...statically assign your IP's...router with 192.168.0.1/255.255.255.0 then make all your PC's 192.168.0.2 / 3 / 4 .... 13, but make 192.168.0.1 your gateway and DNS.

IF that fixes the problem then your DHCP on that router is the culprit
 
the latest firmware for my router is only one day newer than current firmware, and ive been using it for many months with no problems, but i will try that too

i was going to try static IPs too eventually, been meaning to do that anyway...

your idea of mac address is oooooooooooooOOOOOOOOOOO

i just updated bios of BOTH these boards, and problem is only occuring since then

AND i remember at bios download page there was a note about running some patch if u lost mac address.

good call- that might be it. i cant check right now though cuz one computer is running scan on a dead HD that will take some hours (see thread in general hardware forum 🙂)

~Zippy!
 
It COULD be the MAC address, but if the NIC didnt have one, that PC would never work...MAC is layer 2 and IP is layer 3, if no MAC...then the switch would not know where to send to the IP if I am not mistaken. Wouldnt be able to build the ARP table of what port that PC was on.
 
It might be using a default MAC or something that Windows is automatically assigning or a MAC that the bios update wrote the same to both machines, something absolutely silly like that.

The result could be exactly what he describes. Sometimes being able to connect and sometimes not. If each machine is sending at the exact same time, then they'd both fail, but if one sent traffic and then didn't for a few moments (chat programs aren't continuously sending and receiving), then the other one sent traffic, the switch could rebuild the MAC address table to point to the other port each time they do that. Just enough to let a small amount of traffic through, but never enough to do anything useful. And the computers wouldn't see anything wrong, because they'd still have an active network connection.
 
you could try to "net send" to the IP on both those machines...see if it gets there everytime or every other time. This really sounds like an IP conflict.

Yes, Evermore...the above is possible if a MAC is present...
 
both computers DO have the SAME mac id. thanks peeps for all your help, i would have never thought of that!

ill run the ECS fix later today and tell you what happens

~Zippy!
 
Originally posted by: Lord Evermore
Hmmm...just to be certain, look at ipconfig again and see whether both machines happen to have the same MAC address...it'd be odd, but it could happen. The MAC address is writable on that board and is often lost with certain BIOS updates.
Click to expand...


WOW! Nice catch on the duplicate MAC addresses Lord Evermore.
 
Computers tremble in fear when I approach. Really, my roommate hates me for it. I just touch something and it starts working when he's been troubleshooting it all day.

You'd think I'd be getting paid lots of money, but sadly not.
 
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