Multiple network connections: assigning different clients?

Question

At the school where I work, there are two separate networks performing different functions: one to handle district communication (let's call this the district network) and one for the general school population to use. Whenever we have a user that needs access to both networks, we've usually just had to assign them two different computers, one to access each network (cluttering up their workspace), or have them physically switch ethernet cables between one network's drop and the other. Is there a way to put two network cards in a computer so that it's part of both networks simultaneously, and, more importantly, assign clients/applications to specifically utilize one network rather than the other (e.g., logging on the school's Windows domain, web browsing and email would be over the "general" network, while certain financial and database applications would work over the district network)?

Thanks in advance.

Answer 1

You can use two network cards in the same PC, but getting one computer on both networks is the simple part. You will also need to understand the network addressing scheme used for the two networks. If both networks are local, private and physically separate, there is a good chance that both networks are using the same set of IP addresses. For example, both networks may have a host (ie. computer) with IP address 192.168.1.1. If you attach another computer to both networks, it would now be faced with two computers with the same address.

Since the two networks are separated, you may also introduce security issues/breaches by attaching a computer to both networks. Attaching a computer to both networks may permit anyone on either network to access computers on the other network, even though they are not directly connected to the other network. (ie. you probably don't want the general population to have any type of access to the "district" network)

So, yes, in principle, what you are asking is possible. However, given the nature of your question, you should consult with the school's I.T department before making any changes to the network.

Answer 2

What you are looking to do is called "dual homed hosts".  Setting up dual homed machines is actually not all that difficult and most larger networks have at least some hosts or servers set up this way.  As long as the two networks are using different subnets, there should be no problems.  Also, make sure that you determine which is the main route out to the internet.  You might set up a "triangle route" situation if you're not careful.  This simply means that you connect to another machine/server through one interface and the return traffic for that connection comes back on the other interface.  This is not typically a problem but some stateful routers or firewalls may block this.  The risk of "leaking" traffic between nets is actually not really much of a risk.  Most OSs, unless it's an old SunOS box, does not enable IP routing by default.  Even if it did, almost all networks use static default gateway configs rather than dynamic routes via RIP/IGRP etc..  That means nobody would know to use one of these dual homed hosts as a gateway if you don't advertise their IPs.