extend network IP address pool

If you are using DHCP to create IP addresses, then you need to change the mask there and reboot your computers so they will pick up the new mask.  You might want to first shorten the DHCP lease time so when you change the mask, the computers will pick up the new mask more quickly.

If you have static IP addresses assigned, the mask for them will have to be manually changed.  If all the computers don't have the same mask, you can get some odd behavior.  The mask determines the 'gateway' address which is normally 192.168.0.1 and the 'broadcast' address which is normally the highest address in the range.

The mask determines the 'gateway' address which is normally 192.168.0.1 and the 'broadcast' address which is normally the highest address in the range.

The mask actually determines just IP address range in which gateway resides, not gateway itself since gateway can be any IP address from that IP address range (need to be configured). On the other hand - broadcast address is always the last IP address from IP address range (automatically calculated), but for that reason subnet mask must be the same on all hosts.

So according to that, default gateway address can stay the same (it will be still reachable in your case), but on all hosts subnet mask need to be changed.

The rest of Dave Baldwin's procedure how to change IP address range is according to best practice.

Thank you for your answers.

If I understand it correctly,
1st I need to change in the DHCP
2nd any of static IP (Servers, Switches, Firewalls, workstations…) needs to be change manually.
Do I needs to make the change immediately on the static ip address ?
Can it be later ?
 

Kind regards,
David Dotan Sofer

You cannot extend (or change) the DHCP IP pool's subnet mask, it is fixed. You can change starting and ending addresses, and all of the options, but not the subnet mask of the pool itself. You have to destroy and recreate the scope (you cannot set up a second one which overlaps with your current), using the broader subnet mask. Of course you are loosing all options, reservations and leases that way.

You cannot extend (or change) the DHCP IP pool's subnet mask,

That depends on DHCP server itself - on Cisco routers for example (if used as DHCP server) you can actually just change subnet mask without destroying and recreating whole pool, but sure that part could be DHCP server dependent and yet, I guess for Microsoft server can be scripted (powershell script) .... or some other method to make it fast and reliable...

Of course, I assumed DHCP server is a Windows OS, and no networking device. The ability for changing the scope's subnet mask certainly is device/software dependent.

I also remembered that on some routers (can't remember vendor) subnet mask < 24 can't be configured at all.

Just a suggestion: use addresses in the range 172.16.0.0 - 172.31.255.255, for a /16 network. AFAIK using a /24 private network using a mask of 255.255.254.0 is illegal.

No, nothing is illegal. You can use whatever you want to, as long as you stay inside the private ("non-routable") IP subnets. You can even use public IPs on your LAN, but that causes other issues ...
And /24 and a mask of 255.25.255.254.0 are a contradiction - it is /23.
More, subnet classes are outdated. CIDR with /x notation (and x not restricted to 8, 16, 24) is recommended.

Ah, I misunderstood https://en.wikipedia.org/wiki/Private_network :

192.168.0.0 - 192.168.255.255           65,536            192.168.0.0/16 (255.255.0.0)

So the mask is correct. I always thought that it was fixed at /24. So sorry... And thanks.