I just recognised, that I ignored your last part about how to set up the Network on two computers. This wasn't intentional.
Generally, the setup without a distributing device is exactly the same like you do it with a switch inbetween.
Network setup can be irritating and difficult at times. To cover all scenarios and possible issues, would be to much for this thread.
But as I don't know about your basic network knowledge and as well for everyone who wish to know, here is a "short" recap:
Probably most people know, that ip addresses are needed. Every device has to have a distinkt address (IP Address), which must not exist a second time in the same network. All devices which needs to communicate with each other, has to be in the same 'range' of Addresses. These ranges are defined by the 'subnet mask'.
You need to know these addresses, to set them accordingly in the software like Isadora to reach them. (There are other options, but these won't work for all actors/protocols, isadora uses.)
There are (generally) three posibilties to address this.
1. Have them set up automaticaly.
This is the default setting for most OSes and is called Automatic Private IP Addressing (APIPA)
When connected to a network (another computer, or a distributing device like a switch), the OS sets an IP in the range 169.254.0.0 - 169.254.255.255. It automaticaly checks, if an IP allready is occupied and will prevent using it a second time.
The issues with this:
No controll at all about the IP settings. Therefore the need to find them. How easy this is, depends on the OS and other factors.
The IPs might change with every reboot, or even reconnect.
Internet won't work at all in most cases.
Most probably no interaction with computers that has set their addresses in a different way.
2. Using a device which is managing the settings centrally. This service is called DHCP (Dynamic Host Configuration Protocol).
Most routers provide this for easy home LAN and internet connection setup. This DHCP service provides all settings needed by the requesting device and keeps a list of IP adresses, which it gave to the devices.
Most devices which use APIPA, are actually first sending a request to the network, to see if an DHCP service exists. If there is one present, they get the needed settings from it. If they don't get an answer, they go on with APIPA the way described in 1. .
This is working fine for the usual home or office networks, but:
Issues with this:
Control is only managable with the DHCP Service itself and the knowlegde about how to handle it.
By default, IP addresses come with a lease for a set amount of time (usualy 3 days). If the device is not connected in the lease time again, it might loose this ip and will get a new one.
If there are two of those DHCP services present in the same network, they will most likely clash (putting a router in an allready existing network, like in the venues, will most likely have this issue).
Combining this setup with manually setup addresses, needs knowledge about the DHCP server settings (which is the other potential issue, integrating into venue networks).
3. Set a static ip manualy in the settings, provided by the operating system.
Because of the issues listed above, I would strongly recommend going this way. Even if you don't have alot of knowledge about network principles.
Biggest issue: The need for knowledge about the basic IP addressing principles.
This might seem counter intuitive. But with these basics you can tackle issues, which you could only manage with even more diving into network tech when using the other seemingly easier options.
I strongly suggest to have a dip into this with the help of other pages, like this.
At least the parts 'What Is an IP Address?", "The Two Parts of An IP Address" and "The Subnet Mask", will get you going. "DNS" and "The Default Gateway Address" is needed for internet connection, but even if you won't need internet, setting a default gateway will help with issues on Windows, as it takes it as the basis for firewall and other settings.
Other possible issues (There might be a lot more, but these are the most common):
Firewall settings: In generall firewalls and antivirus apps should be put off. At home and most other closed network environment situations, the router will handle this anyway. Firewalls are mainly needed in unsecure situations, like open wifi or other unknown networks.
Port adresses: Protocols like OSC, ArtNet, NDI are using a specific IP port. Every IP address holds more then 65000 possible ports. Some protocols like HTML or certain email protocols, use officially defined ports. If two programms (or actors) use the same port on the same computer, often one of them is blocking it completly. Depending on which one took it first, the following apps/actors can't use them and won't work properly.
Some protocols like NDI, use a "sub protocol", to anounce them self on the network. This is meant to help find compatible devices on the network, without knowing the exact IP address. Some managed switches prevent this "shout outs" to propagate, if not propperly set up.
This discovering method might even work, if the IP adresses are set up wrong. Which might confuse the user, as getting the actual payload (e.g. NDI videostream) still isn't going to work properly, dispite the fact, that the devices are listed as beeing available.
These possible issues are only described in their basics. In reality, they might be even more complex. Like NDI isn't using only one port, but multiple dynamicaly. There are different types of basic network protocols, like TCP and UDP on which are the application protocols (OSC, NDI, etc.) are build on. Specific protocols, like ArtNet are using so called Broadcasting, which can lead to flooding the network, making it very slow.
Beside that, there might be other issues, especially regarding Windows, which sometimes is very confusing about how it handles networks (Keyword here is "private vs. public network", see "Gateway settings" above).
If computers use more then one network card (even wifi in parallel to cable ethernet), one need to make sure, the apps are using the right one. Some offer a setting for it. Isadora e.g. doesn't. Which IP (and therefor network) is used by Isadora, is shown at the lower edge of the 'general' Preferences pane.
This all might seem to be alot to swallow, but with the basics about IP addresses called above, the most cases should be easy to handle.