Network Troubleshooting 101

The following article is intended to provide troubleshooting guidance for basic networks where 1 router and 1 or more switches are used for communication. I provide industry best practices as well as my own experience and expertise.

Basic Network Components:

ISP Broadband Modem
Router with WAN and LAN interface
DHCP Server
Ethernet Switch
Devices (computers, servers, phones printers, etc)

Brief Description:

ISP Modem: Is the device provided by your Internet Service Provider (ISP) that allows you to connect to their communication lines to get internet access. Usually cable providers do the wiring installation and install a cable modem at your premises. For services such as DSL you might have to provide the modem yourself. Another variation would be the Smart Jack your ISP installs in the Dmarc when using T1 lines.

Router: A router is a device that directs traffic to the appropriate destination. It is normal for a small business router device to perform other functions such as DHCP, Firewall, WebContect Filtering, etc. For the sake of this article we’ll keep routers doing their basic routing function and the DHCP service.

Ethernet Switch: Or simply a switch, it’s the device links the network devices.

Devices: anything that connects to the network.


Step 1. Verify the Problem really exists.

Make sure that the problem is real and not a user or application issue. I’ve seen this countless times when a user complaints about the network is not not working and it’s simple as simple as the user never clicking on the send button to send an email.

Step 2. Identify the Problem.

Is it a communication issue with only 1 device, is it happening to just a handful of users, is it happening to everybody in the company?

Step 3. Isolate the Problem.

Once you have identified the problem concentrate on the affected area. For instance if it’s only one user having network issues start troubleshooting at the local level before you move to other areas.

Step 4. Troubleshoot: Cover your basic such as:

Level 1
•Verify the Network connection is enabled.
•Make sure the network cable is connected (if the user connects wirelessly make sure the user is connected to the correct network). Ethernet card display flashing led lights when connected to the network.
•Verify the device is connected to the network switch ( you usually see flashing led light on the port associated with your device).
•Verify the Ethernet cable works, cables do go bad for many reasons.
•Connect to device to a working switch port.

Level 2.
•Verify the Ethernet card is configured with an IP address. For windows you can use the ipconfig command, for linux you can use ifconfig. If your IP address starts with 169.254 the device is not able to reach the DHCP server.
•If you have a valid IP address (similar to other devices in the same area) try pinging your default gateway. If it replies successfully and that device is the only one not able to connect then you might have to check the routing table to make sure things are good there. I’ve seen this happening as a result of a virus and as just normal malfunctioning. For Windows computers you can use the arp -a command to look at the arp table.
•Once you have verified the above and still not connection you might be dealing with a bad Ethernet card. Yes, Ethernet cards do go bad.

Level 3
•I’m leaving this for another article but you can start looking at the router’s settings, DHCP, firewall rules, DNS, etc.

The key to successful troubleshooting is “keeping a systematic layering process” and “Documenting your findings.” These guidelines are not written on stone but provide an essential path in the troubleshooting process. Good luck in your network troubleshooting!

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