Class B subnet 192.168.x.x network

We currently have a 192.168.0.x network with the mask.  We are needing more IP addresses.  I was wondering why I cannot simply change the subnet mask to and have all of the additional addresses that provides.  My consultant says that I would have problems on the network if I did that.  I am running the normal devices on a network (AS/400, servers, PC, Ethernet switches).

Am I missing something?  I know that I would need to change DHCP scopes to reflect the new subnet mask as well as the firewall etc.  I am referring to the big picture... Why would it not work?
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By definition, 192.168.x.0 are Class C subnets that use masks.
Changing to a mask of is technically a supernet mask and no longer fits the definition of a subnet.
Generally speaking, with the new paridigm of CIDR or Classless networking, it doesn't make any difference what mask you use.
However, there are some legacy devices (perhaps AS400) that simply do not recognize anything except a true classful subnet mask and will not work properly with a supernet mask.
Even if it would work, it is generally accepted best practice to keep the subnets managable and within a reasonable number of hosts on a common broadcast domain. Opening up the subnet mask, and therefore adding more hosts, increases your broadcast domain and could have a detrimental affect on your network.
The most practical method is to create multiple vlans, each vlan a separate subnet, and route between the subnets using a fast layer 3 switch. This would be my recommendation. Saves you having to touch every single device that has a static IP address, saves headaches in the long run, and makes for a more efficient network.
You apparently have around/fewer than 250 hosts to be running out of IPs, so I think you should give it a try. You're not at a size, I believe, that needs to worry about dividing into little subnets and routing between them. You may not even have  a route capable of that.

Though you have a larger *potential* broadcast domain with a /16 network (, what matters is the number of hosts you have on that subnet, and 500 is the number that I (and many others) consider to be the point where you should be dividing things up, if your only concern is performance. You can also divide things up in separate subnets/VLANs for security reasons, but you don't indicate that is a priority.

Keeping it simple also has some long term benefits, as far as headaches are concerned and otherwise. Network gurus enjoy all the complexity that can come with complex sub-netting, but it doesn't matter much anymore. You'll know pretty quickly whether it's working OK, and your justification for wanting to try it is sound.

Ideally, everything that can get an IP via DHCP (with a reservation or otherwise) will make the job easier, as it just requires a restart to change the subnet mask settings. Keep a good list of all the hosts that have a static IP assigned so you can get them all changed over, and so you can make sure to get them all changed back in the event that the AS/400 throws a fit.

Do this all after hours, obviously.

Good luck.

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You dont need to do anything with the subnet. Keep the /24 net mask and simply create a new subnet/vlan.

So if your existing network is create a new VLAN and address it as Setup inter-VLAN routing, either on the switch (if it is capable of IP routing) or create a "router on a stick" with an existing or new router.

This is the simplest and also the best approach if you have close to 250 devices on the current network.
Setting up a new vlan (really setting up two where none existed) is not necessarily "simple." Creating a "router on a stick," setting up inter-VLAN routing, or getting a new router is not necessarily the best route. Even though multiple VLANs are probably in the future, making it part of a planning process with equipment procurement is advised.

For what it's worth, lrmoore, I use /16 subnets on three of my vlans which are secure (no routing between them). I use 172.x.0.0/16, where x is also the vlan number. It just makes things easy, even though I don't need that many hosts. Network performance is just fine.

As a test for you this morning, I set up a laptop with manual IP address and manual dns servers. My subnet has a gateway of and dns server So I set the laptop IP to with gateway and dns above (which are in that IP range, though not technically in the same subnet). It works just fine. I am able to connect to all the servers and printers in the range to even though their mask is different. The internet works fine. My point is that since you are changing from a /24 to a /16, the transition should be fairly painless, even if you forget to change some subnet mask settings; this laptop represents what you have now, my larger subnet what you are going to switch to.

Obviously, leave all servers and printers and the gateway in the range until everything is changed over. You might also ask your consultant to be specific about what problems you might have.

Good luck
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