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Ping Problems

Michael McGuire
Michael McGuire asked
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I have a engineer out in the field having problems with comunications with computers out in the field.  I had him send me a detail of his problems with communications.  Here is his problem with the pinging between the computers.:

The server (1) is running server 2008r2, its address was 1.37.44.10,255.0.0.0.
The Siemens PC (2) (PLC) is Win XPsp3, its address is 1.1.37.44, 255.0.0.0.
My PC (3), also WinXPsp3, has an address of 1.7.47.101, 255.0.0.0.

Using the assigned numbers,
  1 could ping 3, and 3 could ping 1.
  2 could ping 3, and 3 could ping 2.
  However, 1 could not ping 2, nor could 2 ping 1.


This is a simple network in a manufacturing environemnt with no internet; DHCP servers; AD; DNS server, ..etc.  Just setup so that these few computers can communicate with each other.  Can anyone see what the problem might be?  I am not onsite with this PC so troubleshooting it has been a problem.
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try a tracert and check where the communications stops
Michael McGuireSystems Administrator

Author

Commented:
Could you please give an example, from the details I have shown on my post, on how to use it.  Sorry don't have any experience with it.
go to the command prompt and
type in "tracert 1.1.37.44" then enter minus the quotes from 1
Michael McGuireSystems Administrator

Author

Commented:
One thing I should mention is that there are not any routers on this simple network.  Not sure if that matters or not for the tracert command?  
it shouldn't matter the command will still work
Thomas AamodtNetwork Architect

Commented:
Also double check the subnet mask if its not 255.0.0.0 it will not work.
if its on different network you need to add routes manually inside of cmd. when not using a router.
It is so typical that the subnet mask would be 255.255.255.0 then that could be an issue.  Sometimes it's even entered by default automatically and one must change it.

What you didn't say is if there is a gateway address entered on any of these computer network interfaces.  That and the subnet mask will make a difference!!

But, let's see:

#1 1.37.44.10  
#2 1.1 37.44
#3 1.7.47.101

If the subnet mask is correct on all of them and if there's no gateway address on any of them, then it should work.  I would still say to turn off all the firewalls just in case as it often is the culprit......

The possiblities are a bit hard to assess.  The thought is this:
- IF there's a gateway that they all point to then there has to be some mechanism for the gateway to NOT drop the packets which is related to its own subnet mask.  In that case one would have to pick an IP address for the gateway, with some subnet mask, that would cause it to drop packets destined for either #1 or #2.
- IF there's not a gateway but all of the computers have a gateway address entered, then if the gateway doesn't exist the packets will be dropped.  That seems unlikely unless.. one of the existing computers is listed as the gateway of another.  In that case the packets would not be dropped going out and, with some luck would be put back out onto the wire at the gateway machine and reach their destination.

Let's say that #2 has #3 as its gateway. and #3 has a subnet mask of 255.240.0.0.
And, let's say that no other computer has a gateway address entered.
Then
#2 could ping #3
#3 could ping #2
#1 could reach #2 with a ping but would not get any returns because the returns would hit #3 and be dropped because #1 isn't in its subnet.
#2 could not reach #1 with a ping for the same reason.
#3 pings would reach #1 if they could get out on the wire .. not sure
#1 pings would reach #3 but not vice versa

So, that's not a matching case but I hope you get the idea.