2 questions in 1 about network subnets

Hello - I have 2 questions about subnets.  1) I would like to understand and get guidance on how exactly to add another subnet to our existing network, for the purpose of expanding our IP addresses - since we are almost out of our Class C 254 IP scope (192.168.1.x).  Once a 2nd one is added, I will move some devices/computers over to it, to free up our first scope.  2) I need to understand how to allow 2 subnets to communicate over the same network.  I am not sure if that means I need to add another VLAN, or if there is another method.  

This is a Windows 2008R2 Server LAN, on a Cisco Catalyst 3850 backbone.

Damian GardnerIT AdminAsked:
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andreasSystem AdminCommented:
Just make your network. So all devices can keep the IP, you just need to change netmask. And then you can have 510 devices on your net IPs from to

This way all hosts can chat directly to each other you dont need a router to connect both subnets.

Drawback you have the broadcast background of 510 devices in your net, which might be undesireable if you have your LAN spanned over some weak low bandwith links.
AkinsdNetwork AdministratorCommented:
I would like to understand and guidance on how exactly to add another subnet to our existing network
Your current scope is (192.168.1.x). Let's assume you DHCP server is
If that's on a switch (lets assume that is using the default vlan 1)
Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#int vlan 2
Switch(config-if)#ip address
Switch(config-if)#ip helper-address

If on a router.
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#int fa0/0.1
Router(config-subif)#encapsulation dot1Q 2
Router(config-subif)#ip address
Your best bet is to create sub-interfaces
int fa0/1.1
encapsulation dot1q

I need to understand how to allow 2 subnets to communicate over the same network
on the switches those devices connect to
Switch(config)#int fa0/1
Switch(config-if)#switchport access vlan 2
% Access VLAN does not exist. Creating vlan 2
%LINK-5-CHANGED: Interface Vlan2, changed state to up

Make sure ip routing is running or turn it on (usually on by default on L3 switches)

Both vlans will appear as connected routes and traffic will traverse between them seamlessly.
Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
nice advice akins, looks a bit advanced for this situation though.

Would it not be more practical to make a second lan interface on the firewall if its a small network?
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andreasSystem AdminCommented:
Even without vlans you can speak as much protocols and IP subnets on one physical LAN as you wish.

So a second Default GW on your Firewall/Router for the new Subnet would also work, but this way you need to renumber some of your devices.

If you extend the (/24) subnet to a 255.255.254 (/23) subnet you just need to change subnetmask.

If you already distribute all IPs via DHCP (even the static ones via IP-MAC mappings in the DHCP) you would be finish in minutes.
Don JohnstonInstructorCommented:
If you are unable to expand the existing IP network space by reducing the mask, I would add a second network to overlay the existing network (AKA Multinetting).

To do this, add a secondary IP address to the routed interface of the device.

For example, if the existing network is routed by the 3850 on VLAN 1, then create the second network with the command:

int vlan 1
 ip address secondary

Open in new window

Then add new devices to the network and move existing devices at your leisure. Once all the devices have been moved off the old network, you can delete the 192.168.1.x IP address from the 3850 and make the address the primary address of the network.

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Fred MarshallPrincipalCommented:
So, the advice is not an "additional subnet" but a "larger subnet".  I agree and the guidance already given is good.

Then, in that case there won't be "another subnet" to communicate.  
But, even if that were the case, you can run multiple subnets on the same *copper LAN*.
Often, LAN and subnet are synonymous - but that need not be the case.
One LAN, as andreas points out, can handle more....
Damian GardnerIT AdminAuthor Commented:
Sorry gentlemen - I posted this right before a major project at the company (which I'm still immersed in), so its taking me some time to respond.  Thank you for your responses here, and I'll reply shortly.  Thanks for your help so far.
Damian GardnerIT AdminAuthor Commented:
I guess I never checked back on this.  My utmost apologies to everyone.  Thank you to all for your suggestions and helping me understand it a little better.  Going to spread the points around.  thank you!
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