Network Troubleshooting at the Command Line – CompTIA A+ 220-901 – 4.4


When you’re
troubleshooting a network, some of the best tools available
are at the command line. So, in this video, I’ll
take you through some of the more popular network
troubleshooting tools that you can find in almost
every operating system. If you’re wondering if a device
is available on the network. Or if it has failed
or been turned off, one of the best tools
you can use is ping. This is going to test,
not only the reachability of that device, but we’ll
be able to tell a round trip time in milliseconds between
our device and the device we’re pinging. This uses a protocol called
Internet Control Message Protocol, or ICMP, to be able
to send this ping request and to get a response back
from that remote device. Ping is one of the most
common troubleshooting tools. It’s often the
first tool you will use to be able to tell if a
device might be up or down. This is a utility that’s
been around since 1983. And it’s certainly one
of the most used network troubleshooting tools available. I’m going to use a Windows
command line to demonstrate ping, but the ping command
is available in almost any operating system. The most basic ping
command might be to type the word “ping”, and
then the IP address, or fully qualified domain
name, of the device you would like to
communicate with. And you can see that
it pings four times, it gets a reply each time, I
get a round trip time value. It even tells me that
it sent four packets, it received four packets. So it lost none of
the packets that was sent between me and
the Google DNS server. When you sit down at a device to
begin network troubleshooting, it might be useful to understand
the IP configuration settings for the device you’re using. Two commands that can
provide you this information is ipconfig if and ifconfig. These are going to provide IP
information for your local IP address, your subnet
mask, your default gateway, and some additional
IP details as well. You would use ipconfig
if you’re running in a Windows environment. And if ifconfig is appropriate
to run in Mac OS10, or Linux environments. In Windows, to see this
information, we use ipconfig. And it shows me all of the
different ethernet adapters, wireless adapters, and
other network interfaces that might be used in
my operating system. For each one of
these, it’s going to provide me with the
IP address information. So for my ethernet connection,
you can see my DNS information. You can see that my IP
address is 10.1.10.25. And the subnet mask
is 255.255.255.0. This gives me also the
default gateway information of 10.1.10.1. There’s other information
available as well. One good way to see that is to
use the same ipconfig command, but use /all at the end. And by adding the /all, we get
much more information about the IP address configuration. I’ll go right back to the top
where it says ipconfig/all. It tells me the host name of
this device, the primary DNS suffix, and other IP details. And now, look at the
difference in information we’ll get for the
Ethernet adapter. Not only the DNS
suffix, but I also get information about the
adapter itself, and the Mac address, along with additional
IP information as well. Let’s look at similar
information in a Mac OS10 environment. This is similar to the command
line, you might have in Linux. And we’ll simply use
the ifconfig command. On my computer, I have many
different network interfaces. You can see them listed
along the left side. If I scroll back up, we can
start to see more details. The primary interface on my
computer is the en1 interface. And you can see an IP version
6 address, and an IP version 4 address. 10.1.10.13 is the
address on my device. So by using ifconfig
and ipconfig, I can learn a lot more about the
IP configuration of the device I’m using. When you’re communicating with
devices across the network, it might be a web
server on the internet, your traffic has to hop through
a number of different routers on the way to that web
server, and back again. It’ll be interesting if we
could see exactly what path it took between my device and
the device on the other side. That might be very
helpful if we feel that there may be some
type of communications issue between my computer
and the computer I’m trying to talk to. One command they can give you
this information is traceroute. This is a command
that’s going to be able to show you in Windows
with the tracert command. Or in a Linux, a Unix, or Mac
OS environment with the command traceroute. The traceroute command
relies on the ICMP protocol. This was the same protocol
we used with ping, but it’s using a
subset of that protocol to be able to determine
what the next hop might be. It uses a functionality
called time to live to be able
to determine this. Time to live is not referring to
seconds, or minutes, or hours. This time to live is referring
to the number of hops that it takes to go
between point A and point B. When we send a packet
across the network, it starts off on our
computer with a large time to live value. This time to live value is
decreased by 1 every time it passes through a router. If a time to live
equals zero, the packet is dropped by that router. And a message is
usually sent back to you called an ICMP time
to live exceeded message. In normal communication, this
time to live exceeded is bad. It usually means that
there’s some type of loop on the network. But for troubleshooting
with traceroute, we can use this
capability to determine what the next path might be. So we set the time to live to
one, we send a message out, and the router that responds
back will be the first hop. Then we send out a message with
the time to leave equal to 2. And the second router will
be the one that reports back with the second hop. This happens over,
and over, and over, again until we’ve built
an entire map between us and the remote device. One challenge with this
is that not all devices will respond back with ICMP
time to live exceeded messages. And some firewalls
will automatically filter out any ICMP. So sometimes traceroute
is available, sometimes it will not be available. It’s going to depend on how
your network configuration is, and what’s between your
device and the device you’re trying to perform
the traceroute on. In Windows, we run
the traceroute command with tracert. And if I type that
in and hit Enter, we’ll simply get an overview
of the different options for the traceroute command. And you can change a lot
of different parameters for trace route. You can tell it not
to resolve addresses, you can specify the
maximum number of hops, and other options as well. In our case, let’s
perform a trace route without any of those
additional options. I’ll run tracert,
and then I’m going to use the IP address of
the Google DNS server. When I run this,
you’ll find it’s going to perform one hop, and
then the second hop, and then the third hop. And you’ll see it build
the path between my device and the Google DNS
server on the other side. In my case, it
took about a minute to perform this trace route. You can see that each hop along
the way is tested three times, and then it gives you either
an IP address, or a DNS name for the hop that you found. And, you can see, mine
goes through Tallahassee, it goes through Georgia, and
then finally finds its way to the Google DNS server. So I now know that there are
11 hops between my device and the Google DNS server. This would be useful
later if there happens to be a problem
with the communication. The traceroute may fail
somewhere in the middle and I’ll know exactly
what the last router was before I was not able
to communicate properly to Google’s DNS server. Another common network
troubleshooting utility that you’ll find in many
different operating systems is netstat. Netstat stands for
network statistics. And although the name is similar
across different operating systems, you’ll find
that some of the options may be a little bit
different depending on what OS you’re using. We’ll run netstat in
a Windows environment where we could run
a netstat -a to show all of the active connections
that are being made into, and out of, our computer. We have netstat -b that
shows this information, and also shows the
application that’s being used, because it will
show the binary that’s in use. And netstat -n is one we can
use to get rid of the names and simply show us IP addresses
in the netstat command. Back on our Windows computer,
if we run netstat -a you’ll see a list of all of the
IP addresses that are in use. The 0.0.0.0 is our local
internal IP address. You can see there are
some port numbers that are listening on our computer. And if I scroll back up,
you can see that I also have outgoing communication that
appears to be going over HTTPS. There’s both TCP
and UDP protocols being listed in this
amount of communication between my computer
and somewhere else out on the network. Let’s run netstat -b to see the
binaries that might be in use. And what I’ll do is
go to my browser, I’m going to refresh
this page so I know that we’ve got
some communication being used by Internet Explorer. And we’ll use the
netstat -b command. It’s going to look at
the communication that’s going inbound and
outbound from my computer. But more importantly, it
adds on to these IP addresses the name of the executable. The binary with the -b. You can see, indeed, that
iexplore.exe, or Internet Explorer, is listed
in communications out to the network. When we ran netstat
-a we were able to see all of the active
connections, but you’ll notice there were delays while
your system was performing DNS lookups so that it
could provide you with the names of these servers,
rather than the IP addresses. If you’re not interested
in the resolve names, or you’d like to see the
IP addresses instead, you can run the same
netstat command with the -a, but add a -n onto the end of it. And it will run the
same netstat command, but instead of waiting for
any type of name resolution, we’re able to instantly see
the communication going back and forth and see IP addresses
listed instead of those resolve names. Microsoft uses the NetBIOS
protocol inside of TCP/IP to be able to communicate
with Windows devices across the network. One way to query these devices
is with the command nbtstat, or NetBIOS statistics. This is going to
not only show us what’s happening on
our local devices, but we can also query
devices across the network. To see the names of the local
device you might be using, we can use the nbtstat command
with a dash lowercase n to list out all of the local
names on your computer. If you happen to
have an IP address of a device across
the network, you can query that device
with nbtstat dash capital A, and the IP
address, to find out what the NetBIOS names might
be on that remote IP address. If you have the
reverse situation, where you have the
name of the device and you’d like the
IP information, you can use nbtstat dash
lowercase a, and the NetBIOS name of that device
to find out more details about that machine. Let’s run an nbtstat
on my local device with the -n to see
the local names. And we can see that this
device is GATEROOM-PC1 and it is in a group called SGC. That’s the name of the domain
on this Windows Network. We happen to know
some IP addresses that we’d like to query. So let’s perform an
nbtstat -A, and we’ll use the IP address
of 10.1.10.225. That’s an IP address
I happen to know on this network, that happens to
be an active directory server. And the name of this device
is specifically SGC-AD01. You can see the other services
that are running on this device as well. If we happen to have that name
instead of the IP address, we can do the reverse. We’ll run an nbtstat -a
and use the name SGC-AD01 and receive exactly the
same information as we did before, except
this time we didn’t need the IP address to be
able to get that information. One very useful Windows
utility is the net command. Net has a number of different
sub commands associated with it. You can see all of
them listed here. So depending on what your
network troubleshooting task is, you may
have a net command available to help you
determine whether you can connect to one of the remote
devices over Windows network. Here are some of the
more common net commands. If you would like to stop
one of the Windows services, you can use net stop and
then stop the service, such as net stop spooler. To start the service, you’d
use the net start command and start that service up again. The net use command is used to
map a drive letter to a share that’s on a different device. We can use net
use, and I’m going to use h:, and then the server
name, and the share name in the Windows standard format. There’s also a net
view command that would help be view
network resources to see if there are any shares
available on a remote device. I know that I have a
print spooler running on this computer, so I’m
going to use net stop and then spooler. And that will stop
the spooler service. If I’d like to start the
spooler service again, I can use net start
and spooler, and it will start the net spooler. You could, of course, use the
Windows utility for services to be able to find
the spooler service and click in the
graphical user interface. As you can see it’s very fast to
do this from the command line. Let’s see if there’s some shares
available on a remote server. I’m going to use
the net view command and I’m going to use a double
backslash, which designates the name of a device. And in this case, it’s
the SGC-AD01 server. This will now show
me all of the shared resources on that server. There’s a netlogon and a sysvol. These are system server shares. There’s also a users
share that I could use. Let’s now connect to this
share of so we can access it. But to do this, we need
to assign a drive letter. If we look at the drive letters
in use on this computer, I’ll pull up the
file manager, and you can see there is an a:,
a c:, and a d: available. I can then choose
any other letter to be able to use
this particular share on the network. So I’m going to use net
use, and I’ll specify m: and then a double backslash
SGC-AD01 to specify the server name. And then a single backslash
and the name of the share. In this case, I’m going to
use the users share name and specify that’s what
I would like to use. And now you can see that the
command completed successfully. I cannot only access this
drive from the command line, I can even go back
to my file manager. You can see now the
M drive is available. And now I can access all
of these folders and files on my local machine,
even though all of them are being stored
on a remote device. In many business
environments we’re able to centrally manage
all of our Windows devices through an Active
Directory database. We can, of course, manage
this Active Directory database from the graphical commands
on our Windows desktop. There’s also a
command line function called netdom that
allows us to perform some of this functionality
from the command line. This netdom command is built
into the Active Directory server. And if you’re running
Windows 8 and later, you can install the Remote
Server administrator tools to enable this
netdom functionality. This allows you to join
a computer to the domain, you can remove devices
from the domain, you can add a domain account,
and many more functions as well. For example, if you wanted
to view information about the domain you’re connected
to, you can use netdom, and the query command with a
/d:, the name of the domain, and then you could specify
workstation, or server, or DC, or OU, or any of these
other options as well. On my test domain I’ve
added a single workstation that we could query. But, of course, on a normal
Active Directory network you might have 10s,
or 100s, or even thousands of devices
in the database. And this is how you’d
be able to view those. Let’s use netdom and we’ll
use the query subcommand. We use a /d: to specify
the domain name. And my domain is SGC. And I’m going to use
the command workstation to list out all the workstations
that are in my database. And you can see there
is one, GATEROOM-PC1. And those are the lists of all
of the different devices that I could find. I can use all of
those different sub commands for the netdom query. For instance, if I wanted to
see the organizational units, I could use OU. And it will spell out the
OUs available in the database and what those
settings might be. We rely on the
domain name system to be able to convert
between names to IP addresses and back again. And if we’re having
problems identifying a particular device,
or you’d like to know what name might be
associated with an IP address, you can use the
nslookup command. This will query a DNS
server and give you information about the
IP address or name that you’re interested in. This is something you’ll find
in both Windows, Mac OS, Linux, and other types of
operating systems. And it’s very flexible. We can use this to
find an IP address. For instance, nslookup
www.professormesser.com will return the IP
addresses associated with that particular, fully
qualified domain name. If all we have is an IP
address, we can still use the same nslookup command. But instead of using the name,
we’ll put in the IP address to see what name is associated
with that IP address. Let’s use those same
examples to run an nslookup. And I’m going to
choose my domain name, professormesser.com. And it’s going to query a server
and find that the answer is 104.20.18.63. There’s actually another IP
address associated with it, 104.20.19.63. And depending on where
you are in the world, I have load
balancers everywhere, so that you might get
different IP addresses listed. But you’ll know that particular
www.professormesser.com is going to be able to resolve
to those IP addresses and you’ll be able to
communicate directly with my web server. Let’s now do the reverse and
find out the name of a device if we have the IP address. We’ll run nslookup again. And instead of the
name, I’m going to simply add
8.8.8.8, which we know to be the Google DNS server. But the specific, fully
qualified domain name is google-public-dns-a.google.com. And again, you might
get a different name depending on how Google has
configured the DNS settings where you happen to be. But you can see how easy
this is with nslookup to either resolver the
name or the IP address, and know instantly what
the configurations are for those different addresses.

21 thoughts on “Network Troubleshooting at the Command Line – CompTIA A+ 220-901 – 4.4

  1. Just wanted to let you know that you are the reason i passed my a+ and network+ exams. Your videos are some of the best study guides on the entire internet, and your cheat sheets that are available on you site are very helpful. Thanks a lot. Please continue making videos and helping all of us looking to continue in our passion of computers.

  2. Ill ping 8.8.8.8 and get replies but when I tracert 8.8.8.8 I get this:

    1 * * * Request timed out.
    2 * * * Request timed out.
    3 * * * Request timed out.
    4 * * * Request timed out.
    5 * * * Request timed out.
    6 * * * Request timed out.
    7 * * * Request timed out.
    8 17 18 17 google-public-dns

    Why would it show stars instead of times?

  3. C:Windowssystem32>netdom query /d:sc.local workstation

    'netdom' is not recognized as an internal or external command,
    operable program or batch file.

    Any ideas??

  4. Curious. When running netstat -b (as administrator on my on personal laptop at home on a network that is in my name) i`m getting 'The requested operation requires elevation' code. Any idea why that might be? Thanks in advance!

  5. Great Videos, sir! Just to add a note for Mac users(and probably Linux users, too): ifconfig in Terminal on the Mac doesn't show you the default gateway/router. Just "ping gateway" will show you the dns name and IP of the gateway and ping it until Ctrl-C.

  6. netstat -b for me comes up as "requested operation requires elevation". I am the only user on the computer however. Anything I can do to solve this?

  7. WOW! thank you, professor, you have no idea how much I needed this,
    I guess I don't have to go through CCNP course any more 🙂

  8. Thank you Professor Messer for the exclusive training videos. I am studying for Cisco CCNA and I find your videos very helpful in general. Any videos on Labs and SIMS troubleshoot? That is where I am having the most trouble.

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