Subnet question

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Jul 7th, 2010
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Hi All,


Just a quick question.


Say I had a subnet of 192.168.100.0/24 with a gateway of 192.168.100.1, but then a requirement comes along to subnet the 192.168.100.0/24 network, into four subnets.


    

     192.168.100.0/24---------------------192.168.100.1--------192.168.200.1----------------192.168.200.0/24




So, I would steal two bits from the host portion and define my four networks are:


1) 192.168.100.0

2) 192.168.100.64

3) 192.168.100.128

4) 192.168.100.192


Broadcasts


1) 192.168.100.63

2) 192.168.100.127

3) 192.168.100.191

4) 192.168.100.255


My hosts would be


1) 192.168.100.1-192.168.100.62

2) 192.168.100.65-192.168.100.126

3) 192.168.100.129-192.168.100.190

4) 192.168.100.193-192.168.100.254



To route the traffic (using a router on a stick concept, and vlan interfaces), would I have to choose an address from the list of host addresses to be the gateway for each of the subnets? i.e.


1) 192.168.100.1

2) 192.168.100.65

3) 192.168.100.129

4) 192.168.100.193


I am pretty sure this is correct, but just want to confirm?


Subnet 1)

Network Address: 192.168.100.0

Range of Addresses: 192.168.100.2 - 192.168.100.62

Gateway: 192.168.100.1

Broadcast: 192.168.100.63


Subnet 2)

Network Address: 192.168.100.64

Range of Addresses: 192.168.100.66 - 192.168.100.126

Gateway: 192.168.100.65

Broadcast: 192.168.100.127


Subnet 3)

Network Address: 192.168.100.127

Range of Addresses: 192.168.100.130 - 192.168.100.190

Gateway: 192.168.100.129

Broadcast: 192.168.100.191


Subnet 4)

Network Address: 192.168.100.191

Range of Addresses: 192.168.100.194 - 192.168.100.254

Gateway: 192.168.100.193

Broadcast: 192.168.100.255



Does this look correct?

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Ganesh Hariharan Thu, 07/08/2010 - 03:35
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Hi All,


Just a quick question.


Say I had a subnet of 192.168.100.0/24 with a gateway of 192.168.100.1, but then a requirement comes along to subnet the 192.168.100.0/24 network, into four subnets.



     192.168.100.0/24---------------------192.168.100.1--------192.168.200.1----------------192.168.200.0/24




So, I would steal two bits from the host portion and define my four networks are:


1) 192.168.100.0

2) 192.168.100.64

3) 192.168.100.128

4) 192.168.100.192


Broadcasts


1) 192.168.100.63

2) 192.168.100.127

3) 192.168.100.191

4) 192.168.100.255


My hosts would be


1) 192.168.100.1-192.168.100.62

2) 192.168.100.65-192.168.100.126

3) 192.168.100.129-192.168.100.190

4) 192.168.100.193-192.168.100.254



To route the traffic (using a router on a stick concept, and vlan interfaces), would I have to choose an address from the list of host addresses to be the gatew


Hi,


Yes you are right as each subnet will act as separet segment so you need to configure sub interface for each subnet like below sample example:-



interface fastethernet0
   no ip address
interface FastEthernet 0.20
   ip address 20.20.20.1 255.255.255.0
   encapsulation isl 20
interface FastEthernet 0.40
   ip address 40.40.40.1 255.255.255.0
   encapsulation isl 40


Hope to help !!


Ganesh.H


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cherian.kiran Mon, 08/16/2010 - 21:43
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Hello Darren,

                      You have subnetted it absolutely right. Since the gateway is also a "device" it will have to get it's ip address from the available subnet. As Ganesh has pointed out, create a subnet interface defining the subnet and feed it with an ip address.


Also, I would prefer to use dot1q more than ISL now ( dot1q provides less overhead and header size compared to ISL). Also, dot1q being an open standard provides for interoperability with other vendors.


One more thing to remember is that the DHCP pool that you will create for your machine should NOT include the gateway address as well.


-/ Kiran Cherian

martin_knorre Thu, 08/26/2010 - 00:48
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  • Bronze, 100 points or more

Hi Darren,


I would also prefer an dot1q encapsulation, but my questoin is, do you use the Cisco router as the DHCP or have you a W2K3 or Linux??

If you have an external server, configure IP Helper on the Subinterfaces and everything should work.


Regards Martin

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