Network Troubleshooting Using The Command Prompt

network troubleshooting command prompt san diego

 

 

Graphic with Network Error

Many network issues can be resolved or at least isolated with the knowledge of just a few basic network concepts. Once the end-user or the troubleshooter has a very basic comprehension of the rules that apply to networking, we can quickly start to pinpoint the problem area in our network. Most of the time network outages occur either at the Local Machine (the computer you are using now) or at the Default Gateway (your wired or wireless router). There are countless approaches as to where to begin the troubleshooting process depending on the type of issue you are experiencing. In this article I will give only a few very basic, but common, network scenarios that cause network disruption of services. To begin with, in order to effectively communicate to the Internet every computer requires four logical components to be configured using TCP/IP settings found under our network adapter properties. Let’s break down the IP address (TCP/IP address or Transfer Control Protocol/Internet Protocol) and explain exactly what its function is and how it works.

The IP address is most commonly described and translated to normal users as being thought of as nothing more than a glorified street address. For starters, in most homes we would assume that the network is using a class c Subnet Mask of 255.255.255.0. This is almost always the case in your network if you are troubleshooting from home or running fewer than 10 workstations on your business network. For the rest of this article we will assume that we are troubleshooting a basic network using a Network Subnet Mask of 255.255.255.0. With this said, let’s now take the IP address of 192.168.1.100 and break it down.

Breakdown IP Address Graphic

IP addresses are broken down into what is called octets. There are four octets in each IP address, each separated by a dot (period). You would thus speak this address as, “192 dot 168 dot 1 dot 100.” The first three octets represents the Network Address, whereas the last octet represents the node address (machine address). The network address portion of the IP address can therefore be thought of as your street address. Think of it as, our office is located on Sunny Crest Lane or (192.168.1.XXX). Our actual building number would then be represented by the .100. My neighbors should all live in the 192.168.1.XXX network if they wish to communicate successfully with me, just as in a network. To keep it simple here every octet can have a number between 1 and 254. Therefore, on my network or neighborhood in this case, I can have up to 254 neighbors all communicating with me.

As I mentioned before there are four components of each machine or node on your network that requires an IP address to be configured correctly in order for your machine to effectively communicate on your network and to the Internet. These four components are your Local IP Address, the Network Subnet Mask, the Default Gateway Address, and your DNS Server Addresses. We already know two of these settings. Our IP address from above will be 192.168.1.100. Our Subnet Mask that is assumed is 255.255.255.0. I know you are probably screaming right now that I said earlier that each octet can only be between 1 and 254, so why is the Subnet Mask reflecting 255’s? For now, the simple answer for this article is, just because. I will cover more advanced subnetting and TCP/IP configurations in later articles. You will also notice that after specifying your IP address manually, that in many cases Windows will automatically fill in the correct Subnet Mask for you. Now on to our last two components.

What is a Default Gateway Address and where do I get one? The Default Gateway on your network is actually nothing more than your router’s IP address. Any time you attempt to access a machine or object outside of your internal network you will communicate with the Default Gateway in an attempt to locate the object. Meaning this, without your router you may be able to communicate with a printer on your network, but you will not be able to communicate with the Internet or check your e-mail without it. Another way to view the router is to think of it as your local post office. If you need to send a letter to someone in a different neighborhood you carry the letter to the post office and they will handle the delivery and confirmation of the letter for you. You just have to make sure it is addressed correctly.

When you purchase a new wired or wireless router it usually comes with initial default IP settings, preconfigured by the manufacture, out-of-the-box. That means that when you power the device on with the cables connected correctly, it should be already configured to act as the Default Gateway for your network with its default settings. To find the manufacture assigned IP address of your router please refer to your router’s user manual, the manufacture’s website, or follow the setup instructions found on the CD normally shipped with your product. I will say this, that most of the manufactures all use the 192.168.1.XXX network with the IP address of 1 or 254. For this scenario let’s assume that the router is a Linksys brand, therefore the default IP address will be 192.168.1.1.

The last component that we need in order to complete our IP address settings is the DNS (Domain Name System) IP address or addresses. The DNS is the service that translates domain names into IP addresses. Think of it as a phone book. Imagine how hard it would be to look up someone’s phone number if they did not have a name associated with each one. In this same way the Internet provides and translates domain names into IP addresses. Instead of typing in 206.190.36.105 into our browser’s address bar, we simply type www.yahoo.com. Although they both work, using names are so much easier to remember. You normally would obtain your DNS entries pushed from your Local ISP (Internet Service Provider) through your cable/DSL modem. For the sake of this article we will use Google’s Public DNS IP addresses. Not only have I not known them to ever go down, but they are also faster in my experience than some locally ran ISP DNS servers.

  • Google’s Public Primary DNS:  8.8.8.8
  • Google’s Public Secondary DNS:  8.8.4.4

If we were manually configuring our IP settings on our local network adapter it would look like this under Network Properties in our Internet Protocol Version 4 (TCP/IPv4) dialog box.

IP Address Settings under Network Adapter Properties

COMMAND PROMPT C:\>_

Our settings are complete, and at this point we would be ready to browse both the Internet and our internal network. Let’s take a look at these settings at the command prompt where we will be spending some time for our basic network troubleshooting. You can access the command prompt many different ways in Windows. Probably the most common is to access it through the Windows Run Command.

Windows Run Dialog Box

  1. On Windows XP Click Start or on the Windows Start Sphere for Windows VISTA OR 7. If you have the newer Windows 8 or 8.1 use the hotkey command, Windows Key + x.
  2. Click on run and type “cmd” without the quotes. In later versions of Windows you can simply type “cmd” on the start menu.
  3. A black text-based dialog window will appear with a blinking cursor.
  4. If you see a window like the image below, you are at the command prompt.

Note that in some cases you should enter into the command prompt with administrative privileges in order to complete various tool commands. To do this browse through the start menu for the Command Prompt shortcut. Right-click on the shortcut and click on Run as Administrator. For this tutorial escalated privileges will not be necessary.

Windows Command Prompt

 

IPCONFIG

From the command prompt we will check out our first tool and probably the most important. It will also allow us another view and chance to verify our network settings to insure proper configuration.

  1. From the command prompt type, ipconfig /all

ipconfig (IP Configuration), is a tool that displays information about your network adapter or adapters. With the tool you can not only check IP addresses, but you can also see the MAC address (Physical Address) of the adapter and whether or not it is configured for DHCP or not. DHCP and MAC addresses will both be discussed in later topics.

ipconfig with duplicate error

In the figure above we can see that our IPv4 address that we assigned this machine is correctly set to 192.168.1.100, however it shows that it is a duplicate. A duplicate IP on a network can cause many issues and in some cases complete network outages. Let’s resolve this by assigning another, unused, IP address.

ipconfig with correctly configured ip address

I simply went back into the adapter settings and changed our IP address from 192.168.1.100 to the new IP address of 192.168.1.200. Remember we can use any number between 1 and 254 as long as it is used only once. Now when I run the ipconfig /all command the <Duplicate> notification is now replaced by <Preferred>. By doing this we just resolved our first network issue and are on our way.

 

PING

The next tool we will see in action is the ping command. Ping is a command that sends an ICMP echo request packet to the node address or domain name you enter at the command prompt. Ping works by first sending the ICMP Request and then waiting on an ICMP Reply from the corresponding node. If ping does not get a reply by the default 4,000 milliseconds or 4 seconds it will time out. Let’s ping our router’s IP address and see how ping works.

  1. From the command prompt type, ping 192.168.1.1

ping command with successful reply

As the figure above shows we successfully ran our ping command. It sent the ICMP Request packet to our router sitting at IP 192.168.1.1. Once the packet request was received by our router it sent the ICMP Reply packet back to our machine to complete the request. It sent the ping four successful times, each taking 4ms to complete. We now know for sure that we can successfully communicate with our network router thus implying that our connection has been established successfully between our machine and router both physically and logically.

Next let’s try pinging an IP address that is not currently in use on our network. We will ping the IP address 192.168.1.163. 

ping command with timed out request

Ah, different results... We sent the packet to the IP 192.168.1.163, but since we did not get a reply from the machine our network adapter responded with, Destination host unreachable. When this happens you may also see from time to time the response, Request Timed Out. I suggest trying to ping a few IP addresses to get the feel for what is actually taking place. Ping is a very powerful tool and probably used on a daily basis for all Network Administrators. Just as a carpenter has his hammer, we have our ping command.

 

TRACEROUTE

TraceRoute is a command line tool that does exactly what it implies in its name. It traces the route that a packet takes to reach its destination host. In windows the command is called, tracert. To run the command you would use the same syntax that the ping command uses. For instance, you could type, tracert followed by an IP address or Hostname. Let’s perform a tracert to the domain hostname, yahoo.com, and see what it looks like.

Successful Tracert command showing hops

In the figure above we can visually see the packet go from our machine, through the Internet, until it reaches yahoo.com. Tracert is defaulted to display up to 30 hops. Every time the packet moves from one network to another it is considered a hop. There may be time outs during the process of tracert, in between some hops; however it could just be that the router that the packet traversed through was configured not to reply to any tracert command requests. We can see though how powerful this tool could be when put into good use. Say we had an office building with three different floors and each floor had its own network. Then we connected all three networks together using routers. We could then shoot a packet from the first floor to a third floor host using tracert. If there was a problem at floor two the results might look like the following given your network design and IP configuration.

Failed Tracert command with time outs

From what we can tell from the figure above is that tracert tried to contact the IP address of 192.168.2.1. Since the network address was not on our network it went to the post office to send the letter out, or our default gateway; which in this case resides at 192.168.1.1. The trace packet successfully hit our router, but oh no... It starts failing at this point, which tells us there may be a connection problem that resides between our router on the first floor and the router that belongs to the second floor.

 

This concludes Network Troubleshooting Using the Command Prompt. For more information on troubleshooting or if you require technical network support please contact our office. We always here to help.



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