Identify Default Gateway on Unknown Network

I thought that the gateway was usually the lowest numbered IP on the network, but in this case, it is not.

Most commonly it is the first usable IP address in the subnet, but it's not required to be. Another fair guess would be the highest usable address in the subnet.

My problem is that I need to identify the default gateway of the network, but none of the PC's on the network have it listed in the adapter settings.

My guess is that none of the PCs are allowed to access the internet. What other devices are connected to the network? You just something that's allowed to access the internet.

Is there a way to identify the default gateway without knowing where the router is physically located or the type of router it is?

Most ideal thing would be if documentation already existing. But it seems that is absent. You could run a network scan. Take the list of addresses where you get responses, then subtract out any that you know are PCs. That should help narrow things down. Many routers have web interfaces. You could try even seeing which devices have port 80 while running your scan.

the first question I would ask is: are you sure there is a gateway running on the given subnet and connecting the subnet to the internet? any known host on the subnet is internet connected?

Originally, none of the devices were allowed access to the outside world.  There is only a 2 PC's, an couple logic controllers and a printer.

Take the list of addresses where you get responses, then subtract out any that you know are PCs. That should help narrow things down.

I thought that is what I was going to end up doing.  Thanks.

you could start at the internet access point that supplies internet and plug a device into one of its lan ports. The site does have internet right?  Also your VOIP controller will have the default gateway. Without a gateway they could be using ANY and I mean ANY (private or public address spaces) for their internal communications.

Originally, none of the devices were allowed access to the outside world.  There is only a 2 PC's, an couple logic controllers and a printer.

But are any devices allowed now? If not, then you're going to be miserable hunting that down. Especially since you don't know where any of the key items physically are, I'd stay open to the possibility that there isn't an internet connection.

Prior to the upgrade, none of the devices on this network had internet access.  I am adding an IP PBX to the network that will be connected to an analog phone line.  

This just occurred to me, all of the devices are connected to a Cisco gigabit switch to allow communication within the network.  If there was no internet access, would there even be a gateway?

Prior to the upgrade, none of the devices on this network had internet access.  I am adding an IP PBX to the network that will be connected to an analog phone line.

What exactly was in the scope of the upgrade? From what you are describing, ot appears that there are still no devices with internet access.

However, since you are utilizing analog lines, internet access isn't an absolute essential for functionality. However, it may become relevant for when the PBX needs updates.

This just occurred to me, all of the devices are connected to a Cisco gigabit switch to allow communication within the network.  If there was no internet access, would there even be a gateway?

A switch doesn't mean there is outside world access, just at least a way to communicate within. If there is no internet access at all, there may or may not be a gateway, depending on how the network is laid out. If there is a gateway, which is generally a router, the gateway wouldn't be able to communicate anywhere outside if there is no internet access.

The scope is to replace the existing Windows XP Towers with Windows 10 Towers.  This included upgrades to the Wonderware HMI, Historian, Win911 Alarm Notification software and XLReporter.  I am not sure if it was required, but the IP PBX is included as part of the Win911 upgrade.  Previous versions of Win911 used TAPI modems for communication.  The new version ditched that technology in favor of SIP/VOIP.

since you replaced the POTS modem (as your connection to the outside world) you now need internet to contact the outside world.

If you need your PBX to connect to a POTS line you need a VOIP Gateway and the PBX needs to know how to work with a VOIP gateways.
The PBX uses what protocols?  SIP/H323/???
You need a compatible FXO gateway..

Here it is explained best i think: https://www.3cx.com/PBX/FXS-FXO/
(most are available for SIP VOIP).

Noci is right about the need for the VoIP gateway. But I will also ask whether you have one or two networks at your location. And also whether there are one or two phone systems.

A VOIP gateway has nothing to do with IP routing... It converts a SIP connection to a POTS link.
Default Gateway is the router on an IP segment. Yours doesn't appear to have it (or need it).
There can be perfectly valid reasons for NOT having an internet connection.  
Be sure you have your VOIP gateway & PBX on a UPS if there needs to be autonomy during power outage.
As they cannot run on power from the POTS connection.

@noci - However, the way that things are laid out at the facility is going to have a heavy impact of how to lay out the VoIP infrastructure (with relation to the phones themselves)

Agree: I think this is key info...

I am adding a Grandstream UCM6202 IP PBX on a network to use for an alarm notification system.  The network was previously closed and did not allow access to outside networks.  Alarm notifications were handled by an analog modem.  The updated alarm software is forcing me to install the IP PBX to allow
VOIP communications

and:

Originally, none of the devices were allowed access to the outside world.  There is only a 2 PC's, an couple logic controllers and a printer.

and:

Prior to the upgrade, none of the devices on this network had internet access.  I am adding an IP PBX to the network that will be connected to an analog phone line.  

There is no apparent need for an internet connections (VOIP doesn't make it necessary). And there can be sound reasons for keeping stuff air gapped.  (Still not bulletproof see Stuxnet) but a lot better then full config on the internet for some cases.

I read this that the original  used a POTS line and that is still should be used....  for this one needs an FXO Gateway)
The PBX does already have such a port on board so it may not require separate hardware. (depends on configuration possibilities).

The local Alarm settings need to contact the PBX then to cause it to call using the right line/number etc.

Sometimes the devil is in the details.

• While we know about this network, is it the only one in the facility (even though we know there is no reason for it to be connected to another network)?
• Is the IP PBX new or replacing an existing PBX? Will the PBX solely cover the area with the SCADA and HMI systems, or does it cover a larger area?

If the organization has 2 networks (one for general corporate stuff, and the airgapped one for factory) and one phone system shared by both, then we have a little bit of a challenge. The replacement phones would either have to be analog (and the PBX would have to be hybrid), adapters would have to be available for that area so analog phones could be used on the extensions in the applicable area, OR extra infrastructure has to be laid out solely for the phones. Nothing that's impossible, just has to be taken into consideration. (I could be over thinking this, but better than underthinking it)

EDIT: I looked up information on the PBX going in. It has FXO and FXS ports, so should be no need for a VoIP gateway. Plus analog phones could be plugged straight into it if necessary.

Apologies for not being able to update this before now.  I also have to be somewhat careful about what information that I post in questions to protect the clients information.  

First, I should clarify some details about the network architecture.  The network consists of (2) PC's running Windows XP and a number of PLC's operating on a closed (air-gapped) network.  The existing alarm notification application, Win911 version 7, dialed out using a MT9234MU Series TAPI modem connected to a POTS phone line.  My project was to upgrade the SCADA software to the latest and greatest.  This required a OS upgrade to Windows 10.  TAPI modems are no longer supported on Windows 10.  This is where the Grandstream UCM6202 IP PBX hardware came in to play.  The network is going to remain air-gapped, and the UCM6202 takes the place of the MT9234MU TAPI modem.  The set up wizard for the UCM6202 required the default gateway, but since there is no internet connection, I was able to just enter XX.X.0.1.  There ended up being some issues with the inbound and outbound routes that had to be addressed, but that was the issue and resolution.  

Thanks to all of you for your help.  I'm going to mark this complete and split points between all of you.