How can I connect two virtual machines in vSphere ESXi 4.1?

There is documentation available for version 5 (I am running 4.1 per the title), but following the directions raises the error:

Incompatible device backing specified for device '0'.

upon clicking 'OK' to apply the settings. I don't really know if creating a serial port connection is necessary in this case, so I'm open to other options. All I really need is to ping the other VM and get a response.

I am new to networking, so the following information may or may not be helpful:

• I have two virtual machines I want to connect, and each are RHEL 5.9.
• They reside on the same vNetwork, but there is no physical switch associated with the vNetwork. There are unfortunately no more physical switches I can use to my knowledge but, if this is indeed the problem, I can see what I can do to get another. Obviously, virtual solutions are 'easier', not to mention cooler.
• I actually lied; I need to connect three machines A, B, and C (all RHEL), however A and C should not be able to contact each other. (If A<B means server B serves client A, then A<B<C.) Thus, I think any solution connecting two machines should scale to this scenario, but I am new to this game.

If you would like more information, please comment and I'll do my best to provide it. :-)

The link you gave is talking about VMWare WorkStation, which should be quite different from vSphere ESXi you use, let alone establishing a serial link need not mean you could 'ping' each other.

Back to the question, I remember if you connect no physical switch to the vSwitch, the VMs connected to that vSwitch could still connect to each other. So, what you are doing should be OK. However, in that case, there will be no DHCP server so you need to either assign static IP to each of the VM network interface, or put a DHCP server on that vSwitch.

If you don't want to allow machine A to connect to machine C, and you don't trust the iptables firewall on them, one of the solution I could think of is have two network interface on machine B, and have each of the interface connect to either A or C, i.e.:

A(eth0)-(eth0)B(eth1)-(eth0)C