Help me access this network device!

July 6, 2016 4:47 PM

Can I access (via http) a device with a fixed IP address in Class B space (172.x.y.z) when it is plugged into my home network that's operating in Class C space (192.*.*.*)?

I salvaged a couple Vorne XL800 Production Monitors (manuals here) and I'm trying to get them working. They apparently were set up with fixed IP addresses (e.g. 172.18.109.49). They have internal servers that one accesses through a browser, but I can't get them to show up in my home network, which uses the DHCP range 192.168.1.64 to 192.168.1.253. I can't see them in any network scan. I can't find a physical way to reset them to factory defaults. Worst case I have to get some software from Vorne that accesses it via RS232 and MAC address, but I'm trying to solve this myself first and maybe learn a little. Can this be done by tweaking the network settings on my computer, router (Arris NVG510), or with a hub or crossover cable? Thanks

posted by achrise to Technology (13 answers total)

Best answer: Can you:

1. Unplug your computer from your home network
2. Plug your computer directly into one of the devices (use a crossover cable if your computer does not have an auto-sensing port)
3. Set your computer to temporarily use a fixed 127.whatever IP
4. Use the browser-based interface to change the IP address on the device to one in your DHCP range
5. Put everything back the way it was before you started

?

This should give you the devices on your home network without any need to have multiple subnets or deal with anything too crazy.
posted by zachlipton at 4:54 PM on July 6, 2016 [1 favorite]

Plug your laptop and the device into a switch or hub (preferably not the one running DHCP), then set a static IP on your laptop NIC to something in the 172.18.109.x range. That should allow you to connect.
posted by tz at 4:55 PM on July 6, 2016 [2 favorites]

You can use an IP alias.
posted by Grumpy old geek at 4:59 PM on July 6, 2016 [2 favorites]

Set your laptop to an IP very close to 172.18.109.49, like .50, assuming nothing else has taken that IP address. It's possible that the subnet mask may be such that the subnets are very small for those devices. This means that there might only be a few IP addresses that can reach them without resorting to routing.

Basically, if the subnet is small, 172.18.109.49.1 or 172.18.109.100 might not be able to reach 172.18.109.49, so use something closer in number.
posted by cnc at 5:02 PM on July 6, 2016

You should grab a RS232 to usb cable, and plug it into your computer and run a terminal emulator.

Fiddle around with the bits per second until some text garbage comes out on the terminal emulator. After garbage comes out fiddle around with the data bits, parity, stop bits, etc. After a while you should find some settings that cause sensible text to pop out. There will probably be a menu that you can adjust the IP address from.
posted by gregr at 5:28 PM on July 6, 2016

I've done this before with printers and other devices. If your computer is Mac OS X, then it's really easy to create a new Network "Location" so you don't have to mess with your normal settings.

1, Go to System Preferences / Network
2. From the Location: popup choose Edit Locations...
3. Make a new Location
4. (Optional) to make things super easy, you can even delete (with the Minus button on the lower left) all interfaces except for Ethernet.
5. Configure your Ethernet using Configure: Manually and like user cnc says above, choose a 172. address that's one off from the device's.
posted by soylent00FF00 at 5:31 PM on July 6, 2016

You can also probably easily change your home network to work in that range. Your router will have DHCP settings that you can make assign addresses in the 172 range. Then everything including the monitors will be able to communicate on a permanent basis. You will have to reboot other network devices for them to get new addresses of you do this. Google for the mannual for your router type for additional instructions. It really isn't hard.
posted by procrastination at 5:47 PM on July 6, 2016

Grumpy old geek's suggestion to use an IP alias is what I'd do - it adds a new IP address/network to your interface, so it doesn't keep you from accessing your regular network like normal. He linked the instructions on Windows. Here's now to do it on Mac OS X.
posted by zsazsa at 5:49 PM on July 6, 2016

Response by poster: Thanks for all the ideas. I'll try them out and report back.
posted by achrise at 6:45 PM on July 6, 2016

If you're curious to learn more -- it really has nothing to do with the fact that one address is in class B space and the other is in class C space. It's that they're in different RFC 1918 private network spaces, which (as the name implies) are reserved for private use, meaning they are not publicly routable. Because of this -- devices in each of your networks do not know how to deliver packets to the other network (at least not without assistance.)

Ordinarily (and I'm simplifying here) your end-user devices know only how to reach devices that are within their own broadcast domain (again, simplifying greatly: which have network addresses that are very close to the ones which they are assigned) and how to reach one or more default routers that are within that broadcast domain -- any traffic not destined for another device within the broadcast domain is sent to one of the routers that your device knows about and it's the router's job to send it on to the right direction.

But because the device you're trying to connect to is in a private address space (which does not have published routes) there's no way for the devices which are supposed to forward the traffic on to deliver it unless you tell them specifically how to reach the 172.* network which is your destination.

People here are suggesting that the easiest way to proceed is to set up your computer to temporarily have an address within the broadcast domain of the destination system and for short-term use they are probably right. The alternative, if you need longer term access, would be to configure your own router to have a static route telling it how to reach the devices in the 172.whatever network (and correspondingly the 172.whatever network will need to have a route back to your 192.168 private network because you have to have traffic able to pass in both directions.)
posted by Nerd of the North at 8:02 PM on July 6, 2016 [1 favorite]

Can I access (via http) a device with a fixed IP address in Class B space (172.x.y.z) when it is plugged into my home network that's operating in Class C space (192.*.*.*)?

Yes you can, simply by adding a suitable 172.x.y.z address to your PC's network interface.

If you're on Windows and you temporarily change your PC's NIC to use 172.x.y.z rather than adding that as an alias, then you'll temporarily lose access to anything on 192.168.1.* (including your router, which means you won't be able to see out to the Internet). That's why it's better to use the alias, making your PC's NIC capable of operating in both address ranges.

172.x.y.z traffic will happily coexist on your LAN with all the 192.168.1.* traffic that's already there; hosts whose NICs have only 192.168.1.* addresses, either fixed or acquired via DHCP, will ignore all the 172.x.y.z traffic. The only time Bad Things will happen from running multiple independent IP address ranges over the same Ethernet LAN is if there are DHCP servers operating in both ranges; DHCP relies on LAN broadcasts and you'd have no way of controlling which DHCP server ended up being used by any given host.

Your Vorne monitors are highly unlikely to contain DHCP servers.

You probably don't want to leave your PC's 172.x.y.z alias in place longterm; better to use the monitor's admin facilities, having contacted it at 172.18.109.49, to move it into a fixed address in the range 192.168.1.1 to 192.168.1.63 (use a subnet mask of 255.255.255.0). This will put it in the same subnet range as all your DHCP-derived devices (which presumably includes your PC) while guaranteeing that (a) you know exactly what address you need to use to talk to it in future and (b) its address won't conflict with anything that got its address from DHCP.

This, incidentally, is why sane DHCP configurations generally don't span the entire range of addresses available in a subnet.
posted by flabdablet at 11:33 PM on July 6, 2016

Response by poster: zachlipton's answer was the easiest and most straight-forward.

Thanks
posted by achrise at 12:34 PM on July 9, 2016

It would also have worked by just plugging the new devices into their final switch ports and using only steps 3, 4 and 5; using a dedicated cable to talk to them one at a time doesn't really buy you anything.
posted by flabdablet at 5:58 AM on July 10, 2016