Subnetting

[nickels85]

I have been asked to create somewhere in the realm of 20 subnets. I understand the concept, but I am having problems implementing it. Having logged in to the switches there are little to no settings that remotely resemble subnetting other than the VLAN's function. I am at a loss. Would I create the subnets on the domain controller which handles DHCP or am I overlooking some sort of function on the switches and or router. Thanks in advance!!!!

-Nick

 

[Schloonce]

Is this what you're looking for?

http://www.learntosubnet.com/

Or do you already know how subnetting works?

 

[spidey07]

You'll have to understand the OSI model for it to become clear. An IP network (subnet, same thing) is considered a single broadcast domain at layer2.

This broadcast domain is a LAN. With switches this is a virtual LAN, VLAN. As far as what defines the subnet that is the job of a router, which can also be a layer3 switch. DHCP or anything AD related has nothing to do with this. It is the network, and more specifically the layer2 and layer3 devices that define the addressing and broadcast domains.

 

[nickels85]

So then considering I have my switches acting as dumb switches with no configuration at all I can go ahead and create all the subnets on the domain controller and the switches will not interfere.?.?. If I provide static IPs for machines on other subnets. I think I have my answer, then. Thanks a bunch guys!

 

[spidey07]

Your dumb switches are a single broadcast domain as themselves. If you connect them together you still have a single IP network (broadcast domain).

AD and DHCP have nothing to do with this. Without a router between these dumb switches you only have a single subnet. The routers define the addressing. Think about what you're doing, it sounds like your in for a world of headache if you don't understand layer2 and layer3.

 

[ITJunkie]

Nickel, I strongly advise you to listen to Spidey...without a router or a switch to do layer3 addressing you won't be able to create your 20 subnets. The DC won't help you with this and all DHCP will allow you to do is to create address pools for the subnets.
You're really going to need to do some studying up on network fundamentals before doing this or, as Spidey said, you will be in for a world of headaches...FWIW.

 

[nickels85]

Hmm... Well I have a Cisco router that doubles as our VPN and it also handles the traffic sent to the switches... I guess my problem is that there is no documentation about setting up subnets on this particular Cisco product which is a router so that makes it layer3. It does however support up to three vlans and one of the switches I have supports up to 50 vlans and is layer 2.... I am still curious how the addressing would work out.?.?

For instance, if I created two vlans on the Cisco router and plugged the switches into those corresponding ports. At that point I would have 2 vlans of 48 ports each. The addressing would still remain the same though right???... So If my DHCP server was serving IP's from .1 thru .254, would those two vlans also only be serving .1 through .254... More importantly if I created additional address pools and assigned static IP's to certain machines would that work???

I realize I am missing a huge piece to this puzzle (I think the dog ate it ), I just really dont know where to start learning so im not chasing my tail. I know the OSI model and understand the difference between layer2 and layer3 and also managed and unmanaged... but something isn't clicking.

Maybe the question I should be asking is for someone to breakdown what hardware is needed for the most simple subnetting scenario? That may make things more clear for me. Again thanks for any and all help!!!!

 

[oddyager]

What models do you have for your cisco gear? You don't really need an actual cisco router per se, any switch with layer3 capability will do as the "router". With what you are trying to do the simplest way would be on that central "router" create a new default gateway for each of your vlans by creating a new virtual interface (say 192.168.10.1/24). Call that Vlan100. Add the ip-helper address command and add in the ip address of the DHCP server (this forwards DHCP broadcasts on that vlan to the ip of your choice). On the DHCP server, create a new scope to distribute 192.168.10.0/24 and exclude the .1 from the pool of course since thats used and anything else (for example if you have static addresses). Make sure in that same scope you designate the new virtual interface you created as the router (this becomes the default gateway for machines connected to that vlan). Then on your floor/closet/etc switches change the vlan of the individual ports that need to be on this new vlan. Rinse and repeat.

I would start reading up on VTP and VLANs (plenty of docs in CIsco to help you with this) and spanning-tree as well before you attempt any of this.