The hub only uses DHCP.
Correct, although the hub might need a reboot in order to promptly request a DHCP address. I have never tested this scenario.
What issue are you trying to address by wanting to unplug the Hubitat from the router? If you need to run it at a location without internet you'd still need a router of some kind (just for the DHCP server and possibly for the NTP server although you can run an DHCP and/or NTP server on an always on Windows machine as well) I used an old WiFi enabled router to do this for about 6 months and used Network Time on a PC as the NTP server. If you just don't have enough ports on the router you can buy a cheap Ethernet switch to connect to the port the hub was connected to then plug the hub and any other devices into the switch. If you want to prevent internet access you can subnet your network with a more advanced router or could run another router as above for offline use. Either way the hub will need an IP which it gets from a DHCP server as you cannot set a static IP on it and that DHCP assigned IP should be reserved for the hub's MAC address so the IP doesn't change since that IP is what you'd use to get to the dashboard or to the management pages.
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If you have simple rules and automation you do not need to have it connected to the router. If you are looking to control it via your phone or tablet, it needs to be connected to your router as your phone or tablet or computer has to be able to connect to it via IP to reach the internal web server of the HE. If you are wanting to forgo that and instead just have pure automated processes then once programed you can disconnect it from the lan. Another option if you're worried about it connecting to the internet is to set up a vlan that goes only one way so it's isolated but your ip connection to it isn't,
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Ok even though the Hubitat hub is connected to the router via ethernet cable, it cannot be accessed via the Internet or WAN?
If the router is connected to the internet and the hub is connected to the router then the hub can and will talk to cloud.hubitat.com and an NTP time server. It gets it's time updates from a NTP time server on the internet if you don't use the community device type to add a local NTP server and it talks to cloud.Hubitat.com to register the hub the first time, get updates on Geofencing info from mobile apps and to make any dashboards you allow cloud access to available to your mobile apps or browsers. The firewall in the router will prevent all other traffic from coming back in from the internet to your hub so the management pages will not be accessible from the internet nor will any dashboards were you disable cloud access.
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Correct, as long as you do not set up and Port Forwarding (on the router) that points back to the Hubitat hub, no one will be able to directly access the hub from the Internet.
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I would like to second this question.
Reading through this thread and the replies you’ve received, it feels like there’s a bit of missing information that would help provide the answers you need to setup the hub in a manner consistent with your intended use case.
But FWIW without any additional clarification from OP, @johnjoe_30 it feels like you are mixing up two (or three) general functions of the hub.
The hub can run z-wave and zigbee networks, with automations already programmed into the hub, and doesn’t have to be plugged into anything but power to accomplish those tasks.
If you want to access the hub’s admin functions to add/remove devices, edit automations or other basic stuff, then the hub will need a LAN connection and a device that functions as a DHCP server. An NTP server would also be needed unless you don’t care at all whether the hub is keeping correct time. You could also get local dashboard access with a LAN-only connection.
If you want any integrations with cloud-connected services, or access to dashboards away from home, or to use the mobile app for geofencing, or you’re unable to provide a local NTP server and need the hub to keep accurate time for your automations, then you’d need a connection to the WAN/internet.
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Hi what is an NTP server and how would i set it up?
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What is NTPD?
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NTP stands for Network Time Protocol. A NTPD would be a NTP Daemon, or a service that provides NTP based time to a client machine (like the Hubitat hub.)
@johnjoe_30 - please explain what your goals and requirements are. It is a little confusing as to what you're trying to accomplish.
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Starting to understand a bit more now!!
So once I've registered the hub my name and email address, set up automations I can unplug the ethernet cable from the router and the automations I have set will run with the Hubitat hub just powered on?
If I want to change any automations, download and setup drivers or devices i have to plug the cable back into the router?
A question I have is quite a lot of my automations are timed for certain parts of the day, so if I unplug the ethernet cable from the router how would the time refresh or stay upto date? I have read some comments about NTP server setup! Not sure what that it is or how to go about setting it up.
I would like to disconnect my ethernet cable when I have to and only reconnect when updating automations so and so forth
When I looked at getting the Hubitat hub, I did what I though was some homework and thought the Hubitat hub was totally free from Internet and router's except for setting up the hub.
I've come from Samsung smartthings platform which was cloud based and something I wanted to get away from.
Hubitat hub seems the ideal solution. I thought the hub didn't need to be attached to the router at all for the phone app to work or the user interface on a desktop computer.
I guess I'm learning this new product to me and struggling a bit.
Hi how do I set that up ?
Yes, but as mentioned above, the hub’s clock will drift, and time-based automations won’t work right.
Yes again. At least with a LAN connection, possibly internet (eg if you’re downloading a hub platform update, which comes from the Hubitat cloud and can only download directly to the hub).
NTP is a very well documented service so I would suggest reading up on that elsewhere then asking additional questions here based on that.
Again, why? 
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Most of us came from SmartThings. I think you're making this a little more complicated than it needs to be.
The Hubitat Hub must have its Ethernet cable plugged into your home network (aka LAN) in order for it to function reliable over long timeframes. The hub needs to be able to get it's time from an NTP server, usually a NTP server out on the Internet. Also, for any sort of user interface capability, the hub must be connected to a local area network.
In no way does the hub rely on the Internet or any Cloud Servers to run its automations. This is the exact opposite of SmartThings. As long as all of your devices are directly connected to your Hubitat hub via Z-Wave, ZigBee, or LAN, the automations will function properly.
If you want to use a Dashboard to monitor and control the hub's devices, the hub must be attached to your LAN to allow PCs and mobile devices a means to communicate to the hub.
If you want to use Amazon Alexa or Google Home (both cloud services), your hub must be connected to the LAN and be able to communicate to the Internet.
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Lol
Correct. This was answered earlier in the thread already.
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Brilliant thank you for clearing that up.
So just to get it into my thick skull, all the automations will work via the hub if the ethernet cable is is unplugged?
It depends on what devices those automations use...and for how long the cable has been unplugged.
If the device is a Sonos speaker that you want to send a Text To Speech message to, it will need the Ethernet cable plugged in (and access to the Internet to perform the TTS conversion using Amazon Poly AWS service 
)
If the hub is unplugged from the network for weeks at a time, there is a good chance the clock will drift and any automation that depends on date/time will no longer run at the correct time. Also, if the hub loses power, and it does not have an Ethernet connection, the time will be wrong immediately after power is restored.
While the hub can be used without an Internet connection, it really does require at least a local network connection to function properly (if any date/time based automations are used.) And without an Internet connection, one would need to run a NTP server on their local LAN, and configure the hub to use that always on NTP server to retrieve the current date/time information. All User Interfaces and admin tasks require a LAN connection to the hub as well.
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Not sure I would put it that way. Nothing is perfectly safe from intrusion as long as it’s connected to the internet (or connected to a unit router, which is connected to the internet).
However I would say that Hubitat is as safe as anything else in your LAN that contains personal and/or valuable information, assuming you follow standard information security practices and don’t take unwarranted risks like port forwarding to your hub like @ogiewon mentioned.
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