Welcome back to your free Windows 7 training course. Regardless of whether
you use IP version 4 or IP version 6 you are going to at some stage need to perform some
network troubleshooting. The first command I will look at is IPConfig.
IPConfig is a powerful command line tool that provides a lot of information about the network
adapters on your computer and how they are configured. If I run the command IPConfig
with no arguments, I can see the basic configuration for each adapter installed on this computer
and also any virtual network adapters. This computer has two network adapters installed
and also VMWare installed. Any virtualization products like VMWare will create additional
virtual adapters. Notice at the bottom you also have the adapters for IsaTap and Teredo.
If I scroll to the top past all the virtual adapters, you can see the basic configuration
details for the two network cards installed on this computer. This will give you basic
information like the IP address or addresses for these network cards and there subnet mask
and gateway addresses. If you need to get more information about
your network cards, you can add the slash all switch. I am also going to add the filter
and more argument. This will pause the display after a screen full of information is displayed.
At the top of the screen the host name is displayed plus other information like the
DNS suffix. The next section will tell you about the physical network adapter installed
on the computer. Some of the information has been cut off so I will press the enter key
to scroll down a couple of lines so all the information for that network adapter is shown.
Besides basic information like the IP address, DNS server, subnet mask and gateway there
is also information like the physical address of the network adapter. This is also known
as the mac address or media access control. If you decide to create a reservation in your
DHCP server so that this computer always gets the same IP address, you will need to enter
in this mac address into your DHCP server. To see the remaining information I will press
space to scroll through one page at a time. The next command I want to look at is
IPConfig slash release. This will give back any IP addresses that this computer has leased
from a DHCP server. If you want to do the reverse and get an IP address from your DHCP
server, run the command IPConfig with the slash renew switch.
If one of your computers on the network changes its IP address you may have a stale cached
DNS record on the computer. If this occurs you can run the command IPConfig with the
slash flush DNS. This will remove any cached DNS records on
the local computer and force the computer to resolve the next DNS request from the DNS
server. If you change the IP address on this computer and you are using dynamic DNS, the
IP address registered on the DNS server for this computer will be incorrect. To fix this,
run the command IPConfig slash register DNS. This will force the computer to register its
host name in the DNS server. Notice the message telling you that the request has been started
but may take up to 15 minutes to take effect. The request should go through straight away,
however if the computer cannot contact the DNS server it will wait and try again later
for a period of up to 15 minutes. The next commonly used troubleshooting tool
is the ping command. The ping command sends a packet to anther computer or device on the
network asking it to inform the sender the packet was received. The device if configured
to do so will then sent a packet back telling the sender the ping was received. Devices
like firewalls often do not response back when pinged.
The ping command works with IP version 4 networks as well as IP version 6 networks. For example
if I ping the IP version 6 address for goggle, the ping command will sent 4 packets to google
in which it will get 4 packets back. The ping command also measures the round trip
the packet took. That is, the time taken to arrive at its destination plus the time it
took to come back. In this case, the DNS record returned will be an IP version 6 record so
windows will automatically use IP version 6.
If I ping dc1 which is domain controller on this network you will notice that windows
will also use IP version 6 to communicate with the server. However, like a lot of other
commands on windows, if I add the minus 4 switch this will force the ping command to
use IP version 4 when pinging the host. IP version 6 would also be used if I added the
minus 6 switch. The ping command by default uses a 32 byte
packet. By measuring the time it takes for this packet to return gives you an idea of
the latency on the network. If you want to test the network with a bit of data you can
add the minus l switch and the size of the packet you want to use. In this example a
4k packet will be sent over the network. When troubleshooting problems on the network
especially networks connected to VPN’s, it is worthwhile testing the connection with
a large packet. VPN encrypted packets and then check the packet on the other end to
see if it is the same. If you have a router in between that cannot handle a large packet
size it will split the packet up into smaller parts. The VPN device on the other end will
think the packet has been tempered with and will discard it. Using the minus l switch
is a good way to determine if large packets are getting through. If a router between you
and the destination attempts to split up the packet the ping will fail.
The last switch I would like to demonstrate with the ping command is the minus t switch.
This is what is called a constant ping. The ping command will keep pinging the other side
until you press control c or close the command prompt. When I reboot servers I like to perform
a constant ping. When the server starts shutting down it will initial still response to pings.
While the server is starting up it will not response to any pings until the networking
on the server has started. Once the server starts up again it will response
to pings again. If you always get pings responses back then the server probably has not rebooted.
Most likely an application on the server will not close and it is stopping the server rebooting.
If the pings stop and don’t return, perhaps there is a DVD in the server and the server
is attempting to boot off it. Either away performing a constant ping lets you know there
was a problem when rebooting the server. When performing a ping and you don’t get
a response at all then you may have a network outage somewhere. To find where this outage
is you can run the command trace route. The trace route command contacts each router between
you and the destination. For example if I attempt to run a trace route
to google dot com, you can see all the routers between me and the destination. As trace route
goes through each router or hop, trace route will attempt to resolve the IP address to
a host name. You will notice that as it goes through each hop, if the IP address can be
resolved to a host name it runs quiet quickly. If the IP address can’t be resolved to a
host name, the computer appears to stall while trace route is waiting for response from a
DNS server. If you want trace route to run faster, you
can disable the resolving by adding the minus d switch. If I run anther trace route you
will notice this time it runs faster as it does not need to wait for a response from
a DNS server. When trace route runs it will test each hop
3 times and record the time taken for the response to come back. If it fails to receive
a response back the time will be displayed as an asterisk. If your packets go through
a firewall, the firewall may block the return from ping and you will get an asterisk in
all columns. The last command I want to look at is the
net stat command. The net stat command shows the active connects from your computer. When
troubleshooting it is helpful to know what connects your computer is making to other
computers. The net stat command also shows the port that
is being used. The last column shows the state of the connection. Net stat is particular
useful if you think that there may be mal ware on your computer. Any connections open
by mal ware will show up here. If you add the minus a switch to net stat
you can see which ports are open on the computer and waiting for connections. This is a quick
way of finding out which services are running on a computer. Finally if mal ware has installed
itself and opened a port up on your computer it will show up here.
This concludes troubleshooting the internet protocol. In the next video I will look at
configuring wireless devices. Remember to have a look at our web
page for more videos, study guides and exam questions.