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Networked Computers Can't See Each Other

Troubleshoot why some networked computers can't see each other, even though they are all able to access the Internet without any problems. Plus: Learn more about the Trace Route tool and its various uses.

By Ron Pacchiano

Q. I have an 8 port switch with a broadband connection and 4 computers. All of the machines have access to the Internet. However, although the machines in LAN ports 1 and 3 can see each other, as can the machines in ports 2 and 4, the two pairs of machines cannot see each other. I've tried using different cables and changing the specific ports the computers are connected to with no success. What could the problem be?

A. There are a number of possible scenarios that could be causing your problem, but without some additional detail it would be difficult to draw any conclusions. However from the information you did provide, we can make some general suggestions which you might find helpful.

For starters, since all of your systems seem to be able to get access to the internet, I would think it's safe to assume that the problem doesn't lie with your hardware. The only exception to that might be if your switch had VLAN features enabled. This would cause the ports on the switch to be segmented into different virtual subnets. In case you're not familiar with VLANs, a VLAN allows network administrators to logically segment a network so that systems from various floors or buildings can belong to a specific virtual subnet (a.k.a. Broadcast Domain) regardless of their physical location. However, VLAN support isn't normally found in SOHO routers, so I doubt that this is the cause of the problem.

More than likely, the problem is due to an improperly configured network setting. It sounds as though your router is not using DHCP to manage IP address assignments, so I have to assume that all of your workstations are using a static IP. You should check your TCP/IP properties to verify that these four workstations are all using the same subnet mask. Typically your router would be configured with a class C subnet mask of 255.255.255.0.

If, instead of this subnet mask, you accidentally configured two of your workstations with a class B address of 255.255.0.0, you would still be able to access the Internet, but not the other workstations on your network. The reason for this is that even with an incorrect subnet mask, the Internet Gateway address is correct. So unknown data packets get forwarded to the gateway like normal. However, the wrong subnet mask would cause your workstations to be segmented differently from the class C machines, thus making them inaccessible.

If that doesn't work, you should check to ensure all of your workstations are members of a common workgroup. Different workgroup names are a common problem in peer-to-peer networks and would make it difficult to access the other PCs.

As a matter of fact, depending on which OS these PCs were running, your problem might have more to do with not being able to access a shared folder as opposed to being able to "ping" a PC as we originally assumed. In this situation the problem would have more to do with a lack of user rights and security associations. Windows 98 PCs, for example, don't have as stringent security concerns to deal with, whereas Windows NT, 2000 and XP workstations do. So you might need to take a closer look at the rights and group settings of the specific folders you're trying to access. Good Luck!

Q. Can you explain the purpose of Trace Route and how it is used?

A. TRACERT is a command line utility which functions in much the same way as the Ping utility does. Ping measures the time it takes for a network packet to reach its destination. TRACERT takes this measurement one step further by not only tracking the amount of time it takes for the packet to travel through the Internet but also monitoring how many hops (or routers) the packet has had to travel through before arriving at its destination.

TRACERT is a particularly useful diagnostic tool for identifying problems between you and your ISP. In companies with multiple sites it is also useful for determining potential problems with your network's telecommunication system. TRACERT lets you see if the routers are operating as efficiently as possible by allowing you to visually watch a data packet being sent and received during its journey through the Internet.

TRACERT determines the path to its destination by sending ICMP Echo Request messages with varying Time to Live (TTL) values. The first Echo Request message is sent with a TTL of 1, which is increased incrementally with each subsequent transmission. This continues until the target responds or the maximum number of hops (the default is 30) is exceeded. The path to the destination is traced by examining the ICMP Time Exceeded messages returned by intermediate routers and the Echo Reply messages returned by the destination. The TRACERT command is part of the TCP/IP Protocol suite.

To use the TRACERT utility, open a command line window and type TRACERT followed by the destination IP address or qualified domain name of the site you want to test. Give it a try by tracing the route to Practically Networked. The program will display the Internet address and response times from each router between your location and Practically Networked. An example of this would be:

C:\TRACERT www.practicallynetworked.com

Some locations (including Microsoft for example) will turn off their response to the PING and TRACERT commands for security reasons.

More information about TRACERT and its operating parameters can be found at http://support.microsoft.com/default.aspx?scid=kb;en-us;162326

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