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In my office network, the DCHP Server is set to 192.168.0.1/24.

I want to increase the available pool, so I am thinking update the above setting to 192.168.0.1/23 in order to have:

Network:   192.168.0.0/23
HostMin:   192.168.0.1
HostMax:   192.168.1.254
Hosts:     510 devices

So, if I am thinking right when I update the setting, a new device will able to get an IP address from 192.168.0.100 - 192.168.1.254. Between 192.168.0.1 - 192.168.0.99 I give static IPs. My question regards these devices which have static Ip.

After the change, If I want to assign a PC with static IP, I will set its settings to:

 IP: 192.168.0.50
 Subnet: 255.255.254.0 //is this value right?
 Gateway: 192.168.0.1
 DNS: 1.1.1.1

But, what will happen to those devices that I have already assign them a static IP? For example, I have a network disk NetDisk where I have configured its address as:

IP: 192.168.0.50
Subnet: 255.255.255.0
Gateway: 192.168.0.1
DNS: 1.1.1.1

So my question is: When I make the change in my router will NetDisk will be disconnected from the network, because of its value of subnet 255.255.255.0?

I am asking because it would be a problem as I will not be able to login and change the settings afterwards.

4

Nothing will disconnect right away, but you'll be running into a problem.

IP nodes sharing a common L2 segment can only communicate directly when they share a common subnet. Remember that each node uses its own IP address and subnet mask to determine 'neighbor' nodes sharing the same subnet.

Accordingly, nodes from 192.168.0.0/23 incidentally using addresses from 192.168.0.0/24 will have no problems talking with nodes being configured directly for 192.168.0.0/24.

However, that's only half your new subnet. Those nodes not within the common subnet (ie. those from 192.168.1.0/24) will talk to 192.168.0.0/24 fine - from their POV they share a common subnet, packets are sent directly and make it. But the other direction - from 192.168.0.0/24 to 192.168.1.0/24 - won't make it. These node see a destination remote from their subnet and they'll send the packets to their default gateway.

You either need to change those nodes' subnet mask to /23, or you need to set up a gateway in between. There are other approaches but they are just more complicated. Whether nodes are configured manually or by DHCP doesn't matter.

The correct order of enlarging the subnet would be:

  1. Change the static nodes to /23 subnet mask. Do not yet use addresses from the enlarged subnet.
  2. Change the DHCP server's and the gateway's bindings to /23 mask.
  3. Change the DHCP scope's mask option to /23. Wait for all leases to renew.
  4. Start using addresses from the enlarged subnet.
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  • Thanks a lot for you detailed answer! – yaylitzis Sep 26 '20 at 12:25
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    "But the other direction - from 192.168.0.0/24 to 192.168.1.0/24 - won't make it. These node see a destination remote from their subnet and they'll send the packets to their default gateway." AIUI whether or not they will make it depends on how the default gateway behaves, some routers will send packets back out the same interface they came in, others wont. – Peter Green Sep 27 '20 at 1:08
  • So there is no way to force the expiration of a DHCP lease early, once issued? – Tripp Kinetics Sep 27 '20 at 1:33
  • @TrippKinetics No, not from the DHCP server side. Of course, you could trigger a lease renew some other way (e.g. running a command by central management). Also, you could considerably shorten the lease time prior to the change. – Zac67 Sep 27 '20 at 7:12

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