How does NI MAX discover RT targets on a different subnet?

[JKSH]

Hello,

I know that for full functionality, my host PC and the cRIO need to be on the same subnet. Nonetheless, NI MAX is capable of detecting cRIOs that are on a different subnet as my host.

I'm curious: What mechanism does NI MAX us to find the the cRIO in this case?

[ARVega]

Hello JKSH,

Try this:

1- Right-click on Remote Systems, select "create new"

2- Select "Remote Device (not on the local subnet)"

3- Follow the prompted instructions. 

Let us know if these instructions helped you!

[joerg.hampel]

In short, communication between the host and the cRIO has to be reliable and stable, so it's done via TCP/IP, a transport-layer network protocol that provides reliable, ordered, and error-checked delivery of data between two parties. This is also called one-to-one or unicast addressing.

Discovering devices without prior knowledge of their number or addresses requires sending a one-to-many message out into the network. This is called multicast (many receivers) or broadcast (all receivers) addressing. TCP/IP clearly doesn't support this communication scheme. The most common transport layer protocol to use multicast addressing is UDP.

There's a PDF giving a little insight into the mechanisms that MAX uses to discover devices:

http://digital.ni.com/public.nsf/$CXIV/ATTACH-AEEE-9Y9P55/$FILE/Network%20Ports%20Used%20for%20Devic...

Google any of the protocols and mechanisms, there's an abundance of information to be found.

[joerg.hampel]

Just stumbled upon a very nice explanation of "how packets get around": 

http://www.informit.com/articles/article.aspx?p=131034&seqNum=5

[rtollert]

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@JKSHI'm curious: What mechanism does NI MAX us to find the the cRIO in this case?

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To more directly answer your question: it doesn't.

MAX is not capable of spontaneously discovering controllers on other subnets, because as others have pointed out, discovery is fundamentally a link-local process — it doesn't cross network gateways. It can only connect to remote targets if you give its IP, or its hostname if it's registered in DNS.

FWIW, this has a couple of esoteric implications for target formatting and reimaging. If you reformat a controller on the local network, and its IP address changes, MAX is able to identify the new IP address based on its link-local broadcast announcements. But if a remote target's address changes — i.e., it's addressed by IP and its IP changes, or it's addressed by hostname and its hostname is  changed or is removed from DNS — then you're out of luck.