Multi-homed XP

[Rob]

I have an XP pro computer with two network adapters, each
connected to a different network segment. Call the
segments A and B. I want to be able to ping a computer in
segment B from a computer in segment A, going through the
XP computer as if it were a router. All I have succeeded
in doing so far is being able to ping the XP machine from
either segment, as well as ping network segment hosts
from the XP machine. I have played with the routing table
ad nauseum but I cannot see how packets can be directed
from one interface to the other on the XP machine. What
am I missing?

 

[Steve Winograd [MVP]]

I have an XP pro computer with two network adapters, each
connected to a different network segment. Call the
segments A and B. I want to be able to ping a computer in
segment B from a computer in segment A, going through the
XP computer as if it were a router. All I have succeeded
in doing so far is being able to ping the XP machine from
either segment, as well as ping network segment hosts
from the XP machine. I have played with the routing table
ad nauseum but I cannot see how packets can be directed
from one interface to the other on the XP machine. What
am I missing?

Enable IP forwarding on the multihomed computer to cause packets from
one interface to be repeated on the other Interface. Create routes on
all of the other PCs that define how to use the multihomed PC as the
gateway to the other segment.

For example, assume that:

Segment A uses the 192.168.0.0/24 subnet.
Segment B uses the 10.0.0.0/8 subnet.
The multihomed PC's network cards are 192.168.0.1 and 10.0.0.1.

Then these routes are needed on the other computers:

Segment A: route add 10.0.0.0 mask 255.0.0.0 192.168.0.1
Segment B: route add 192.168.0.0 mask 255.255.255.0 10.0.0.1

If the network doesn't use the Internet, you could avoid the subnet
routes above and instead:

Segment A: set the default gateway to 192.168.0.1
Segment B: set the default gateway to 10.0.0.1

Computers on each segment will be able to access the other segment by
IP address. If you want them to be able to access computers on the
other segment using computer names, then each computer needs an
LMHOSTS file defining the names and IP addresses of the computers on
the other segment.
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Rob]

Thanks, Steve. I am happy to say that I came to the same
conclusion last night, even using the same subnets as in
your example, once I stopped staring at the multihomed
computer's routing table and focussed instead on the
routing tables of the computers in the subnets.
Incidentally, I scoured microsoft.com for discussions on
multihomed machines but I could not find one article that
discusses the multihomed machine in the context of the
network. That is, nowhere do you find a discussion of
what needs to done vis-a-vis the routing tables in
machines trying to communicate through the multihomed
machine...as you have done in your reply to my question.
So, we need to get a kb article into the knowledge base,
appropriately indexed, so that the neophytes like me can
more quickly see the forest for the trees...so to speak.

-----Original Message-----

 

[Steve Winograd [MVP]]

Enable IP forwarding on the multihomed computer to cause packets from
one interface to be repeated on the other Interface. Create routes on
all of the other PCs that define how to use the multihomed PC as the
gateway to the other segment.

For example, assume that:

Segment A uses the 192.168.0.0/24 subnet.
Segment B uses the 10.0.0.0/8 subnet.
The multihomed PC's network cards are 192.168.0.1 and 10.0.0.1.

Then these routes are needed on the other computers:

Segment A: route add 10.0.0.0 mask 255.0.0.0 192.168.0.1
Segment B: route add 192.168.0.0 mask 255.255.255.0 10.0.0.1

If the network doesn't use the Internet, you could avoid the subnet
routes above and instead:

Segment A: set the default gateway to 192.168.0.1
Segment B: set the default gateway to 10.0.0.1

Computers on each segment will be able to access the other segment by
IP address. If you want them to be able to access computers on the
other segment using computer names, then each computer needs an
LMHOSTS file defining the names and IP addresses of the computers on
the other segment.

Thanks, Steve. I am happy to say that I came to the same
conclusion last night, even using the same subnets as in
your example, once I stopped staring at the multihomed
computer's routing table and focussed instead on the
routing tables of the computers in the subnets.
Incidentally, I scoured microsoft.com for discussions on
multihomed machines but I could not find one article that
discusses the multihomed machine in the context of the
network. That is, nowhere do you find a discussion of
what needs to done vis-a-vis the routing tables in
machines trying to communicate through the multihomed
machine...as you have done in your reply to my question.
So, we need to get a kb article into the knowledge base,
appropriately indexed, so that the neophytes like me can
more quickly see the forest for the trees...so to speak.[/QUOTE]

You're welcome, Rob, and congratulations on figuring it out --
definitely not a task for a neophyte.

The multihomed computer doesn't need added routes because it has an
interface with an IP address in each segment, which automatically
creates the needed routers.
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com