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Subnetting

Posted on 2003-11-20
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Last Modified: 2008-02-26
Hi I am really confusing in subnetting as more and more i go through books.
My basic question is when subnetting is done( i know bit). It is basically splitting the main net work.say
192.168.2.0
subnet mask 255.255.255.192
This will give 4 subnets
 (192.168.2.0, 192.168.2.64,192.168.128 and 192.168.2.192.)
Lets say these network id's and host  will be assigned by DHCP.
Now my question is how many NIC i need and where those will be hooked up. Does it need four NIC or just one (for 192.168.2.0, because we are just splitting this).
How it works in real environment. or it is done through L2 switches. Is subnetting works at Layer 2 or layer 3. Whole lot of confusion, in theory it look quite good. How a host is connected throuh, is it splited through Hub/swiches. Layer 2 0r Layer 3 switches.where the NIC is from where ip address is coming.

Some simple diagrame showing NIC (important) and subnet will be appreciated.
regards and thanks

jatsw
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Question by:jatsw
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NicBrey earned 500 total points
ID: 9786098
Hi
IP addressing is Layer 3, so is subnetting.

There is more than one way to do this.
The best way to do this, is with a layer 3 switch. Then you need only one NIC in your server. the switch is more expensive, but performance will be better.

Create 4 VLANs on the switch and configure the routing between them. A VLAN for each subnet.
The DHCP server can be in any one of the subnets with a static address for that subnet.
Configure the 4 DHCP scopes on the server - one scope for each subnet.

Depending on the type of switch you use, the implimentation might differ slightly.  In the Cisco world, you add the
"ip helpder-address <ip address of server>" command on each of the VLAN interfaces. Without this, your layer 3 device that handles the routing between the subnets will not forward the DHCP requests to the server.

Will look something like this...

Subnet 1
(with DHCP server)--------------\
                                              \    
Subnet 2 -------------------------\  \
                                                Layer 3 switch
Subnet 3 -------------------------/     /
                                                 /
Subnet 4 --------------------------/

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Expert Comment

by:bartt
ID: 9816794
Hi Jatsw

What NicBrey is suggesting is good. You need tot create 4 VLAN's and connect them up to a layer 3 switch. Because of the nature of DHCP (broadcast to get the IP-info) it is needed to make clear to the VLAN which is at Layer2 where the DHCP server is.

Yes Subnetting is a layer 3 thing. If you look into the OSI model for IP, IP is layer 3 related. Layer 2 is just the MAC-address.

+----------------------------+
|    Application layer        |
+----------------------------+
|   Session layer              |
+----------------------------+
|    Network layer            |  => IP related
+----------------------------+
|   Datalink layer              | => everything related to the MAC
+----------------------------+
|   Physical layer              |
+----------------------------+

If their is no need for the VLAN's to interconnect I would be surprised that there are layer 2 switches which are able to set up a kind of IP-helper address.

If you like that your devices also can have contact Inter-Vlan you have to set some routing in the switch. I don't know how you do at the Cisco boxes (I hate them, not stable) but with an Extreme Networks switch it is just an <<Enable ipforwarding vlan vlanid>>
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LVL 7

Expert Comment

by:NicBrey
ID: 9816870
Hi Bartt,
Guess the choice of different hardware vendors is a matter of personal preference.     I am a Cisco man...    :)

>If their is no need for the VLAN's to interconnect I would be surprised that there are layer 2 switches which are able to set up a kind of IP-helper address.

Layer 2 devices split collision domains. Layer 3 devices split broadcast domains. You do therefore not require a command on a layer 2 switch to forward DHCP requests to the server. Broadcasts gets forwarded out all switch ports. It is only needed when the DHCP server is on a different subnet, seperated by a layer 3 device.

Nic
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