Adding router with internet connection to existing subnet

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jalovegren

Mar 12, 2018
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Hi,

I have an isolated subnet right now which contains several devices on fixed subnet (let's call this Subnet A) over wired ethernet. One of the devices is the DHCP server (let's call this "Master"). None of that can be modified--these are pieces of commercial hardware which are hard-coded to be on this particular subnet. I have a laptop which is also connected to this subnet. This laptop behaves as kind of a mobile GUI to visualize data on this subnet.

Now I'm trying to add internet to the picture, with the goal being that I can connect to the internet from the laptop, while at the same time still being able to monitor the commercial subnet. How should I go about doing this?

My plan was to add a router, and have the internet come into the WAN port of the router. Then, I would set the subnet from the router to match Subnet A, then disable DHCP on the router. The internet and gateway would then be from the router, while the DHCP is still from the Master device.
Would this work? If so, can most home routers support this? I'd like to be sure before I buy equipment.

My only other idea is to add a router with two VLAN subnets, one for 192.168 (Subnet B) which would be used for devices that need internet, and the other subnet being Subnet A. However, with the laptop on Subnet B, I don't know how I'd be able to talk to the devices on Subnet A.

Any advice is appreciated!
 
Solution
B
Your best bet it is to just pick a IP that is not used and set it static on your pc and on your second router. Even if you can't change it you should be able to look at the dhcp server config and see how it allocates ip. Most have a range excluded from the pool and you can use any of those or just pick a very high address in the pool. The dhcp server will start allocation at the bottom of its pool In most cases it never will hit the top. Also most dhcp servers attempt to ping the IP just in case for example the DHCP server would get rebooted while machines were still active it try to not give out the same ip.

You can not have 2 dhcp server and even if you tried fancy multiple router solution you would still need a static ip...
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B

bill001g

Titan
Aug 9, 2012
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It should work to use consumer router as you describe. I would assign a unused but sorta random ip to the router just in case the current devices have the more common x.x.x.1 set as the gateway. That way only you pc would know the gateway ip address.

The big IF in this equation is some consumer routers limit the range of ips. Some for example only allow 192.168.x.x with standard 255.255.255.0 subnets. Other restrict the lan ip to the private ip blocks only. Make sure to read the manual before you buy to be sure there are no restrictions on what ips you can use on the lan.
 
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J

jalovegren

Mar 12, 2018
2
0
10


I just realized there's another complication: the devices on Subnet A, including the DHCP, are sometimes powered off, but I'd still like to get internet on the laptop. This means that either I need to have separate subnets, over which the laptop can talk to both, or I need the DHCP to be turned on for the router, meaning 2 DHCPs would be active on the same subnet when everything is powered on... thoughts?
 
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B

bill001g

Titan
Aug 9, 2012
28,367
2,766
125,640
Your best bet it is to just pick a IP that is not used and set it static on your pc and on your second router. Even if you can't change it you should be able to look at the dhcp server config and see how it allocates ip. Most have a range excluded from the pool and you can use any of those or just pick a very high address in the pool. The dhcp server will start allocation at the bottom of its pool In most cases it never will hit the top. Also most dhcp servers attempt to ping the IP just in case for example the DHCP server would get rebooted while machines were still active it try to not give out the same ip.

You can not have 2 dhcp server and even if you tried fancy multiple router solution you would still need a static ip on the primary network if the dhcp server is not on.
 
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