Class B subnet 192.168.x.x network

We currently have a 192.168.0.x network with the 255.255.255.0 mask.  We are needing more IP addresses.  I was wondering why I cannot simply change the subnet mask to 255.255.0.0 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?
dkempaAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

lrmooreCommented:
By definition, 192.168.x.0 are Class C subnets that use 255.255.255.0 masks.
Changing to a mask of 255.255.0.0 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.
0
schapsCommented:
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 (255.255.0.0), 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.
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
KlinkeyeCommented:
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 192.168.0.0/24 create a new VLAN and address it as 192.168.1.0/24. 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.
0
schapsCommented:
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 172.20.0.0/16 has a gateway of 172.20.1.254 and dns server 172.20.1.200. So I set the laptop IP to 172.20.1.233/255.255.255.0 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 172.20.1.1 to 172.20.1.254 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 192.168.0.1-254 range until everything is changed over. You might also ask your consultant to be specific about what problems you might have.

Good luck
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Network Architecture

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.