1. Troubleshooting a Network

2. Introduction
Whether you have a small network in your house with only one PC or a large enterprise network with thousands of PCs and servers, troubleshooting a network can be challenging. The complexity of troubleshooting increase with the number of devices and the scope of the network infrastructure.
However, there is an important set of core troubleshooting steps that you would use to troubleshoot a network, no matter its size and scope.

3. Various Steps in Troubleshooting a Network
Know Your Infrastructure
Learn about your network
When Network connection is down
When there is no IP Address
When there is no DNS Server
No Default Gateway
Mis-configured Subnet mask

4. Know Your Infrastructure
While you can blindly perform troubleshooting, you will be much more successful by first knowing your infrastructure i.e. what is connected to what and how it is designed.
If this is a Windows enterprise network, perhaps may be lucky to have a network design. Ideally, you are troubleshooting your own network, that you designed, but we can’t always have that luxury of a design.

6. Learn about your Network
If you don’t know your infrastructure design and there’s no documentation to reference, you can start troubleshooting by learning about your network infrastructure from a few common network diagnostic tools. There is actually a lot to be learned by simply running IPCONFIG and/or the LAN details Windows GUI.

8. Learn about your Network
From these two commands, you learn a lot which includes:
Whether the network interface is up or down (physical network connectivity)
How to obtain an IP address for a node – DHCP or static
Whether a node has an IP address, or not, or an automatically obtained IP address (those that start with to   , or APIPA)
Your default gateway
Whether you have DNS servers configured and what they are
All this information is tremendously valuable in solving your Windows network problems.

9. Network Connection is Down
If you look at the OSI model, the physical layer (layer 1) is at the bottom. If layer 1 doesn’t work then NOTHING else is going to work. It is best to troubleshoot from the “bottom up” (from layer 1 up to the higher layers).
The IPCONFIG command would indicate whether the network connection is up or down.
The LAN details in GUI would also indicate if the connection is up or down

10. Network Connection Down
In the graphic on the right, you can see that the Media State is Enabled. That is the normal, fully-functioning state. However, if the media state is disabled then you need to:
Check the network cable – is it unplugged? Disconnected? Cut?
Check the network switch – is the network cable connected there? Is the port enabled?
You’ve got to get this working first, before anything else will work.

11. No IP Address
Whether you have no IP address ( ) or an automatic private IP address (APIPA, starting with x.x), it doesn’t matter. You’ll have to obtain an IP address before you can use the network. If you are set to use DHCP (likely the default) and your DHCP server is down then that is the reason that you don’t have an IP address.
Options to solve the problem:
Resolve the issue with the DHCP server
Statically assign an IP address that isn’t in conflict, is on the right subnet, etc

12. No DNS Servers
If you don’t have any DNS servers configured then you’ll be able to communicate on the network with IP addresses only. You should be able to ping your default gateway, your DHCP server, and other servers on the network. However, you won’t be able to do any of that by name. Options to solve this problem:
Determine why the DHCP server isn’t providing DNS server IPs
Manually configure the DNS server IPs if you know them
Configure public Internet DNS servers like the Google public DNS servers
On the other hand, if you have DNS servers configured but you can’t ping them then you need to check the DNS servers – perhaps they are hung or packets are being restricted through a firewall.

13. If there is No Default Gateway
If you have no default gateway IP address configured, this will prevent you from communicating on another IP subnet (like the Internet) but not from communicating on your local LAN. Thus, even with no IP default gateway configured, you should be able to work as normal with local servers. Remember, configuring a default gateway is optional if you don’t need to reach an external network.
You could manually configure a default gateway or determine why the DHCP server didn’t provide one for you.
Alternatively, if you have a default gateway defined but you can’t communicate with it (even with a ping) then you need to check your local router.

14. Misconfigured Subnet Mask
If you have an incorrectly configured IP subnet mask then you’ll get some unpredictable results. You either need to manually configure it (if you are using static IP addresses) or check the DHCP server to see why it gave you the wrong subnet mask.

15. 10 Basic Network Troubleshooting Tools
Ping
Tracert/Tracerout
Ipconfig/ifconfig
Nslookup
Netstat
Subnet and IP calculator
Speedtest.net/pingtest.net
Route

16. Ping Command
The most commonly used network tool is the ping utility. This utility is used to provide a basic connectivity test between the requesting host and a destination host. This is done by using the Internet Control Message Protocol (ICMP) which has the ability to send an echo packet to a destination host and a mechanism to listen for a response from this host.
Simply stated, if the requesting host receives a response from the destination host, this host is reachable. This utility is commonly used to provide a basic picture of where a specific networking problem may exist. For example, if an Internet connection is down at an office, the ping utility can be used to figure out whether the problem exists within the office or within the network of the Internet provider.

17. Tracert/Traceroute
Typically, once the ping utility has been used to determine basic connectivity, the tracert/traceroute utility can used to determine more specific information about the path to the destination host including the route the packet takes and the response time of these intermediate hosts.
The command shows you the path a packet of information takes from your computer to one you specify. It will list all the routers it passes through until it reaches its destination, or fails to and is discarded. In addition to this, it will tell you how long each 'hop' from router to router takes

18. Tracert/Traceroute
Figure 2 below shows an example of the tracert utility being used to find the path from a host inside an office to The tracert utility and traceroute utilities perform the same function but operate on different operating systems, Tracert for Windows machines and traceroute for Linux/Unix based machine.

20. Ipconfig/IFconfig
One of the most important things that must be completed when troubleshooting a networking issue is to find out the specific IP configuration of the variously affected hosts. Sometimes this information is already known when addressing is configured statically, but when a dynamic addressing method is used, the IP address of each host can potentially change often.
The utilities that can be used to find out this IP configuration information include the ipconfig utility on Windows machines and the ifconfig utility on Linux/*nix based machines.

21. NSlookup
The nslookup utility can be used to lookup the specific IP address(es) associated with a domain name. If this utility is unable to resolve this information, there is a DNS issue. Along with simple lookup, the nslookup utility is able to query specific DNS servers to determine an issue with the default DNS servers configured on a host.
NSLOOKUP is an application that facilitates looking up hostnames on the network. It can reveal the IP address of a host or, using the IP address, return the host name.
It is very important when troubleshooting problems on a network that you can verify the components of thenetworking process. Nslookup allows this by revealing details within the infrastructure

22. NETSTAT
NETSTAT is used to look up the various active connections within a computer. It is helpful to understand what computers or networks you are connected to. This allows you to further investigate problems. One host may beresponding well but another may be less responsive

23. Speedtest.net/pingtest.net
A very easy test that can be used to both determine the Internet bandwidth available to a specific host and to determine the quality of an Internet connection is the use of the tools available at the speedtest.net and pingtest.net websites. The speedtest.net site provides the ability to determine the amount of bandwidth that is available to a specific host at a specific point in time; this is often a good tool to use when measuring how long it is going to take to upload or download information from a local to remote host. This measurement can also be used to determine whether the connection is offering the amount of bandwidth that was purchased from the Internet provider.
Keep in mind however that some amount of bandwidth difference is expected between the quoted bandwidth purchased and the measured bandwidth. The pingtest.net website is used to determine the quality of the connection by measuring the ping response times and jitter amounts over a short period of time. This information can be used to determine a likelihood of how well the measured connection will deal with certain types of high demand traffic like Voice over IP (VoIP) or gaming.

24. END OF LECTURE