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Making Sense of Windows Routing Tables



Published:
Dec 08, 2005



Updated:
Dec 08, 2005



Section:
Articles & Tutorials :: Network Troubleshooting




Author:
Brien M. Posey



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It’s amazing the way that time can go by. People tend to think of computers as being high tech,
but the TCP/IP protocol has been around in one form or another for over three decades. As such,
TCP/IP has had time to really mature and be made stable and reli
able. When it comes to
computers, nothing is foolproof though. When routing packets across a network, things do
occasionally go wrong. In these situations, it is helpful to be familiar with the Windows routing
tables. Routing tables dictate the flow of pac
kets from the machine in question. In this article, I
will show you how to view the Windows routing tables and how to make sense of the data that is
contained within it.

Viewing the Routing Tables

The routing tables are an important part of Windows’ TCP/I
P protocol stack, but they aren’t
something that the operating system normally displays to the casual user. If you want to see the
routing tables, you will have to open a Command Prompt window and then enter the ROUTE
PRINT command. Upon doing so, you will

see a screen similar to the one that’s shown in Figure
A.


Figure A:

This is what the Windows routing tables look like

Before I delve into the routing tables, I recommend entering another command into the
Command Prompt window. The command is:

IPCONFIG /
ALL

The reason why I am recommending that you use the IPCONFIG /ALL command is because it
shows you how TCP/IP is really setup on the machine. Sure, you could look in the TCP/IP
section of the network adapter’s properties sheet, but the information is more rel
iable if you get it
from IPCONFIG. I have seen a couple of instances over the years in which IPCONFIG reported
completely different information than what was entered into the machine’s TCP/IP configuration
screen. This doesn’t happen often, but if the righ
t type of error occurs you can experience this
type of mismatch. To put it bluntly, the information that’s keyed into the TCP/IP properties sheet
reflects how you would like Windows to set up the TCP/IP protocol for the choosen network.
The information pre
sented by IPCONFIG shows how Windows has actually configured the
protocol.

Even if you haven’t had some bizarre Windows error, it’s still useful to get your configuration
information through IPCONFIG. If a machine has multiple network cards, it can be toug
h to
remember which configuration is bound to which card. IPCONFIG lists the various
configurations in an easy to read, per NIC basis, as shown in Figure B.


Figure B:

The IPCONFIG /ALL displays the machine’s TCP/IP configuration on a per NIC
basis

Examin
ing the Routing Tables

Right about now you might be wondering why I had you to do an IPCONFIG /ALL, when this
article is supposed to be discussing routing tables. The reason for this is that normally you never
even look at the routing tables unless you are

having problems with your machine. If you are
having problems, then the best place to start the troubleshooting process is to compare the
information provided by IPCONFIG to the information stored in the routing tables.

As you saw in Figure B, the IPCONF
IG /ALL screen displayed some basic TCP/IP information
such as the IP address, the default gateway, etc. The routing tables aren’t quite as intuitive
though. Therefore, I want to take some time to discuss how to read the routing tables and what
the informa
tion in the tables mean.

In order to understand what the information in these columns mean, you need to understand a
little bit about how a router works. A router’s job is to facilitate moving traffic from one network
to another. As such, a router will con
tain multiple network interface cards, each connected to a
different network segment.

When a user sends a packet that’s destined for a different network segment than the one that the
PC is presently attached to, the packet is sent to the router. It is up
to the router to figure out
which network segment the packet should be forwarded to. It doesn’t matter if the router is
connected to two network segments or a dozen. The decision making process is the same, and
it’s all based on routing tables.

If you look

at the Route Print screen, you will notice that the routing tables are divided into five
different columns. The first column is the network destination column. This column lists all of
the network segments that the router is attached to. The Netmask colum
n provides the subnet
mask not of the network interface that’s attached to the segment, but of the segment itself. This
basically allows the router to determine the address class for the destination network.

The third column is the gateway column. Once the

router has determined which destination
network it needs to send the packet to, it looks at the gateway listing. The gateway listing tells
the router which IP address the packet should be forwarded through in order to reach the
destination network.

The In
terface column tells the router which NIC is connected to the appropriate destination
network. Technically, the interface column only tells the router the IP address that has been
assigned to the NIC that connects the router to the destination network. How
ever, the router is
smart enough to know which physical interface the address has been bound to.

The final column in the routing table is the Metric column. Metrics are a science in themselves,
but I will try to give you a brief explanation of what they do
. The best way that I have ever heard
metrics explained is in terms of an airport. Imagine for a moment that I needed to fly from
Charlotte, NC (the closest major airport to my home in South Carolina) to Miami, Florida. Being
that the Charlotte airport is
pretty big, I have a lot of choices of how I could get to Miami Beach.
I could hop a North West Airlines flight. It would take me to Detroit Michigan and then down to
Miami (Detroit is a bit out of the way). Likewise, I could hop a Continental Airlines fli
ght that
would take me to Houston, TX, and then to Miami. Another option would be to just take a US
Airways flight nonstop to Miami. So which airline should I take?

In real life, there are a lot of factors to consider such as the price of the ticket and th
e departure
times, but let’s assume that everything was equal. If there were no differences between the
airlines other than the route, then I would fly the airline that makes the fewest stops. It would get
me to my destination more quickly, and since there

are fewer stops, there would be less chance
of having a problem with my connection, lost luggage, and things like that.

Routing works the same way. Many times, there is more than one way that a router could send a
packet. In such a case, it makes sense to

send the packet along the shortest (or most reliable)
path. This is where the metrics come into play. Windows does not even look at metrics unless
there are multiple paths to a destination. If there are multiple paths though, Windows checks the
metrics to

determine the shortest route. This is an over simplified explanation, but it gets the
point across.

Additional Routing Options

Earlier, I showed you the Route Print command, but there are actually a lot of other things that
you can do with the ROUTE comma
nd. The ROUTE command’s syntax is as follows:

ROUTE [
-
f] [
-
p] [command [destination] []

The

f switch is optional. This switch tells Windows to clear the routing table of all gateway
entries. If the

f switch is used in conjunction with other commands then

all gateway entries will
be cleared prior to executing other instructions within the command.

The

p switch makes a specified route persistent. Normally, when a server is rebooted then any
routes that you specify via the ROUTE command are removed. The

p
switch tells Windows to
keep the route even if the system is rebooted.

The command portion of the ROUTE command’s syntax is relatively simple. The command set
consists of four options PRINT, ADD, DELETE,

and CHANGE.

I’ve already shown you the
ROUTE PRINT c
ommand, but even the ROUTE PRINT command has other options. For
example, you can use wild cards with this command. For instance, if you only wanted to print
routes pertaining to the 192.x.x.x subnet, you could use the command ROUTE PRINT 192*.

The ROUTE DE
LETE command works very similarly to the ROUTE Print command. Simply
enter the ROUTE DELETE command followed by the destination or the gateway that you want
to delete from the routing table. For example, if you wanted to remove the 192.0.0.0 gateway,
you c
ould enter the command ROUTE DELETE 192.0.0.0.

The ROUTE CHANGE and the ROUTE ADD commands have the same basic syntax as each
other. When you enter this command, you must usually specify the destination, subnet mask, and
gateway. You might also specify a m
etric and an interface, but that’s optional. For example, if
you wanted to add a destination using the bare minimal syntax, you could do so as follows:

ROUTE ADD 147.0.0.0 255.0.0.0 148.100.100.100

In this command, 147.0.0.0 is the new destination that you

are adding. 255.0.0.0 would be the
subnet mask for the destination, and 148.100.100.100 would be the gateway address. You can
extend the command with the METRIC and IF parameters. Doing so would look something like
this:

ROUTE ADD 147.0.0.0 255.0.0.0
148.100.100.100 METRIC 1 IF 1

The metric parameter is optional, but it specifies the metric or number of hops for the route. The
IF parameter tells Windows which NIC to use. In this particular case, Windows would use the
NIC that’s bound to Windows as inte
rface 1. If you don’t use the IF parameter then Windows
will automatically search for the best interface to use.

Conclusion

In this article, I have explained how to use the ROUTE command to display the Windows
routing tables and make changes to those table
s if necessary. If you need a little extra help, you
can get more syntax examples by entering the ROUTE /? Command.