Troubleshooting Networks – CompTIA A+ 220-901 – 4.4

| December 30, 2015

It’s always the network, and it’s never the network. In this video, you’ll learn how to troubleshoot and resolve some of the most common network issues.

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There’s nothing worse than not being able to communicate to the internet. We need to step through a number of troubleshooting tasks that will take us from the very beginning of the process all the way through determining just how much of the network we’re able to access.

One of the first things we should look at is the physical connectivity. Are we connected to a physical ethernet connection, or is there a link-light that we can look at? That would at least tell us if we happen to have a cable plugged in, and if that cable is indeed connected to a switch on the other end.

A way to check to see if your local TCP/IP stack is configured properly is to simply ping your universal loopback IP address, which is 127.0.0.1. This should respond back immediately because you are pinging your internal device. It doesn’t have to go out to any ethernet connections, to any routers, or any switches. If We’re not able to get a response back from the loopback, then something has gone wrong with the IP configuration of your operating system.

The next step would be to ping the IP address of the adapter that’s connected to the network. This might be a physical ethernet connection, or it might be a wireless connection. This checks the local configuration of your IP addressing, makes sure that the adapter is working properly, and it will confirm that you have a link light. Because if you don’t have a link on that connection, the adapter will not be available and will not respond to a ping.

Now that we can ping an IP address associated with our local interface, let’s ping something that’s also on our local subnet. And a good example of that would be the default gateway. If you can ping from your device to the default gateway, you at least know now that you have connectivity to the rest of your local network.

The next step would be to determine if we can communicate to devices that are outside of our local network. And one way to do this is to try some devices that are well known on the internet. One that I like to use is the DNS service that Google provides, because it’s IP address 8.8.8.8. It makes it very easy when you’re trying to remember a device to ping. I can ping 8.8.8.8, and we can see if it can get a response from communicating all the way to that device out on the internet and back again.

If you’ve turned on your computer and it’s not able to communicate to a DHCP server, it will attempt to assign a link-local address. This is also an IPv4, called an automatic private IP address, or a APIPA. This allows you to communicate with other devices that are on your local subnet. But an APIPA address is not going to be routable, so you can’t communicate outside of your local subnet.

You can look at the IP address of your device to see if you’ve been assigned an APIPA address. If your IP address is 169.254.1.0 through 169.254.254.255, then you have an APIPA address. The exact range for APIPA is a little bit larger than this, but the first 256 addresses and the last 256 addresses are reserved.

Once your computer decides to use APIPA, it picks an address and then sends an ARP out over the network to see if anyone else is using that APIPA address. If nobody else is, it assigns it to your adapter card. If it does find a conflict, it chooses another address somewhere in this range and performs the ARP began to see if it can find an address that’s available.

So let’s have a look at a computer that was assigned an APIPA address. Here’s an example in Windows, and you can see that the autoconfiguration IPv4 address– it even tells you that this is an autoconfiguration address. And indeed it is– 169.254.228.109, which is certainly in that range for APIPA.

If you’re in Windows, and you see a message that you have limited connectivity or no connectivity, you may even see an alert or an icon in your system tray for this, you may want to see if you’ve been assigned an APIPA address. Since APIPA is a link-local address, it will only operate on your local subnet. That certainly qualifies as a limited connectivity.

If Your DHCP server has assigned the address, you may want to perform the normal ping test. Ping your local device, ping your local gateway, ping a remote IP address to see if the problem is on your subnet or if it’s located somewhere outside of your local network.

One frustrating problem you might run into is when everything is working normally, and you’re able to access devices over the network, and then suddenly, the network isn’t working at all. You have this intermittent access, where it’s working, and then not working, and then working again. And this could be a bit of a challenge to try to troubleshoot, because you’re never quite sure when the problem may be occurring.

One thing you can do is check to see what the operating system is showing. If you have a broken LAN icon in your system tray, that may be indicative of a loss of signal. So check your cable. Make sure things are plugged in properly. You may want to even replace the adapter cards you’re using to see if the problem is with the hardware itself.

The problem could be related to what you’re connecting to. There might be a switch on the other side with an intermittent connection. Or you may be connecting to a wireless router that’s periodically rebooting, and you’re losing connectivity while the reboot is happening.

Every device on your network should have a different IP address. If you do have a situation where two devices are using exactly the same IP address, then you’re going to have an IP conflict. Generally, our DHCP servers are able to resolve this because they’re handing out the IP addresses and they’ll insure that there are no conflicts on the network.

But occasionally, you will have a conflict when you’re statically assigning addresses. Fortunately, your operating systems are very good at identifying these by sending an ARP out over the network before it connects to make sure that no device out there is going to respond with the IP address that you’d like to use.

If you do see two devices that do manage to get on to the network with the same IP address, what you’ll find is neither of them have very good access to the network. And they each have intermittent connectivity to communicate out to other devices. One thing you may want to do is to reboot one of the devices, or reset the network interface card, or simply restart the process of obtaining an IP address, that way you could be sure there are no IP conflicts on your network.

If you work at all in networking, one of the biggest complaints you’ll hear is that the network is slow. And unfortunately, there’s no single thing you can point to determine why the end user is seeing some type of slow down. This is not only frustrating for the end user, but it’s really frustrating if you’re the one who has to troubleshoot this particular problem.

One way to attack this problem is to look at each individual component on the network to see how it’s operating. So you may want to look at your routers, or your switches, and see what the performance is of all of those devices. If those devices are overloaded, or your network links are saturated, then you can understand why there might be a slow down at the end user’s workstation.

Sometimes an issue with performance is related to the configuration settings of an adapter card or the ones that are configured on a switch. Ideally, the speed and the duplex need to match on both sides. And occasionally, an autoconfiguration will simply put one side out of sync, and then you’ll have performance problems as they try to communicate with each other.

The issue could also be more of a physical layer issue, where the wiring or the cabling that you’re using is simply bad. Maybe a cable got crimped, or maybe there’s a short in a cable, so you may want to replace the cabling, or the adapter card, and see if that resolves the problem you have with the slow down.

And sometimes, the slowdown has nothing to do with the network and everything to do with how the operating system is performing. Unfortunately, a malware infection can cause performance problems, not just with the network, but with everything else that’s running in that operating system.

If you’re running into problems with your wireless signal, it may be caused by a number of different issues. One might be interference. There maybe other devices that are using the exact same frequency as you, and therefore, you’re not able to communicate well over that wireless network.

Your problem might just be related to the strength of the signal. You may want to look to see what the transmitting signal happens to be. And if you have options to change the signal strength, you may want to look into that. Modifying the receiving antennas and transmitting antennas might also improve your performance on the signal strength as well.

You might also want to look at the channels that are in use on your wireless access point. These are usually determined automatically by the software inside of the access point. But you may be able to manually configure this as well. Some channels might work better than others, it may just depend on what other devices might be in your local area.

Another problem you might see, especially at higher frequencies, is that these signals like to bounce off of the other objects around you. So you may want to change your location or change where the access point is located on your network. And of course, you may want to move that access point, just so you can get the antennas closer to your users. The closer your users are to the receiving antennas, the better the performance is going to be on that wireless network.

Wireless interference can come from a number of different sources. It might come from your fluorescent lights, their microwave ovens, and cordless telephones, and other high-power devices that can cause interference on your wireless network. Ideally, the signal strength of your wireless network is going to be higher than the noise that is created by the interference around you. Sometimes, the source of interferences completely unpredictable. You may be in a multi-tenant environment, either at home or at work, and of course, everybody’s going to have their own access points, running their own frequencies, and you could potentially conflict with those.

One way to measure this amount of interference would be with some specialized software, perhaps you can use the signal strength meters or performance monitor is built into your operating system. Or you can use specialized software, like this one. It’s going to measure the signal strength versus how much noise is going across that network, and ideally, you’re going to have a much higher signal strength than the overall noise on that network.

One feature that we’ve become accustomed to on our wireless networks is listing out all of the available wireless networks in our area, so that we can choose the one we’d like to connect to. But what if you’re not able to find the SSID name, or the network name, that you’re looking for? It may be that the network you’d like to connect to is farther away than you’d expect, and there’s a network closer that’s using the same frequencies that’s too loud. It’s overwhelming the signal that would normally come from your distant access point.

It could also be that the administrator of your wireless network has disabled this SSID broadcast, and it’s never going to appear on this list. To be able to connect to this wireless network, you need to know its name, and you have to manually enter that in your wireless device.

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Category: CompTIA A+ 220-901

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