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Author Topic: Network bridging
littlefish
BlabberMouth, a Blabber Odyssey
Member # 966

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Icon 1 posted February 04, 2009 03:48      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Hi all,

I have a PC (winXP) with two network cards, one connected via DHCP to the network, the other has a static IP and is connected through a crossover cable to a (for all intents and purposes) a webserver with IP 10.0.0.1. Troubleshooting, both connections appear to be OK, but I cannot speak to the server. I think that XP is sending all my traffic out to the LAN (I can connect fine through that NIC). How do I tell windows that IPs in the range 10... should go through the other NIC?

If my assessment of what is wrong is wrong itself, please let me know what is wrong instead. [Wink]

Posts: 2421 | From: That London | Registered: Nov 2001  |  IP: Logged
GrumpySteen

Solid Nitrozanium SuperFan
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Icon 1 posted February 04, 2009 06:39      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
Do you know what a subnet is? If you do, skip the next paragraph.

Two computers communicating via TCP/IP must be on the same subnet in order to communicate with each other. The subnet is the first set of bits in the IP address which correspond to the bits of the subnet mask which are set to 1. For a typical home network, it's enough to know that 255 in the subnet mask means that the corresponding number has to be the same in both IP addresses in order for the computers to talk to one another.

If an IP address is on a different subnet, you will need a gateway which can communicate to both subnets. That means the gateway will have two or more IP addresses and will read messages sent to it on one subnet and retransmit them on the other subnet.

Once a gateway is set, any message the system needs to send to an IP address that's not on the same subnet will be sent to the gateway instead.

(hopefully that helps, even though it's a really skimpy explanation that leaves a lot out)

Posts: 6364 | From: Tennessee | Registered: Jan 2000  |  IP: Logged
dragonman97

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Icon 1 posted February 04, 2009 08:01      Profile for dragonman97   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Um...I'm a Senior Networking Something-Or-Other, and I thought that answer was obtuse, so I fear for littlefish...

People seem to obsess over the term 'subnet' - they're certainly important as a plot device on 24, but IP addresses and gateways are the more important thing to pay attention to (the netmask will follow).

littlefish, here's what you need to know:

Find out the IP & subnet of the webserver.
You already know the IP: 10.0.0.1, but what's the subnet mask?
If this is a quick & dirty setup, it's probably 255.255.255.0.

Then, make sure the XP machine's second network card has the same subnet mask, and a *similar* IP address (that isn't already in use).
For instance, 10.0.0.2 with a subnet mask of 255.255.255.0.

If this is hooked up with a crossover cable, you should *not* set a default gateway for this network card.

What /probably/ happened was that the IP address and/or subnet mask wasn't correctly configured for the second network card, and the computer was trying to reach it through the default gateway (via the first network card).


I don't want to say that subnets are unimportant - far from it...but they're quite a bit to *really* get unless you've worked on things for awhile. (For me, it took listening to my boss for years, and one day I was looking through a netmask calculation HOWTO on a train ride when it all became clear.) Dotting your 'i's and crossing your 't's will get you to the same place without the headache.

--------------------
There are three things you can be sure of in life: Death, taxes, and reading about fake illnesses online...

Posts: 9332 | From: Westchester County, New York | Registered: May 2001  |  IP: Logged
dragonman97

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Icon 1 posted February 04, 2009 08:09      Profile for dragonman97   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
P.S. If the first network card (the DHCP one) is set up with a 10.0.0.xyz IP address and a common subnet, Windows will still opt to go out through this card. Change your binding order to put the second NIC first. You can do this by going to Start -> Settings -> Network Connections. Then click Tools -> Advanced Settings. Move the second NIC higher up in the top box.

--------------------
There are three things you can be sure of in life: Death, taxes, and reading about fake illnesses online...

Posts: 9332 | From: Westchester County, New York | Registered: May 2001  |  IP: Logged
littlefish
BlabberMouth, a Blabber Odyssey
Member # 966

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Icon 1 posted February 05, 2009 09:46      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Well, thanks for your help, but it still isn't working. I'll stick in a bit more info in case it is any help, and seek a live administrator to troubleshoot.

The server is an instrument run over IP and is headless. Stickers on it give the address as 10.10.0.1, with subnet mask 255.255.255.0. The manual suggests the PC default configuration as 10.10.0.100 and 255.255.255.0. This is what is set up.

The other NIC is controlled by DHCP and currently has the settings
IP 130.209.52.176
Subnet 255.255.255.0
Gateway 130.209.52.1

As I say, I have support but thought I should be able to work it out myself. Does having the same subnet mask for both NICs cause problems?

Thanks again for your help.

Posts: 2421 | From: That London | Registered: Nov 2001  |  IP: Logged


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