Adding a 2nd subnet on our network

Damian Gardner
Damian Gardner used Ask the Experts™
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Hello - I just want to get some general guidance on this.  We're at capacity on our 192.168.1.x /24 subnet, and are adding a 2nd subnet to our DHCP server for 192.168.2.x / 24.  We've had Cisco add another VLAN to our network so that both VLANs 1& 2 can talk to each other.  What I need clarity on is when adding the 2nd subnet to the DHCP server, how do I control what clients take addresses from 1 subnet as opposed to the other?  Do I just statically address machines to control that?  Or do I NEED to worry about that at all?  Will DHCP automatically hand out addresses from the 2nd one when the 1st one gets down to 0 IPs left?

Thanks for your help.
Damian
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Ken BooneNetwork Consultant

Commented:
So the DCHP is on your 192.168.1.x network.  This is how it works.  Your DHCP server is listening for two things.

1) Broadcast DHCP requests.  - Since it is a broadcast, the DHCP server knows that it must be from the network that the DHCP server is on because broadcasts are local to the network.  Therefore it gets a DHCP request as a broadcast on the interface on the 192.168.1.x network.  It then responds with an address out of that pool.

2) DHCP relay requests - These are DHCP unicast packets that are sent from a router in order to relay a DHCP request from another network segment.  When the DHCP server receives these packets, the relay packet has the network from where the request is coming in the packet.  The DHCP server sees this and then knows which scope to send an IP address from.

Now to make it work on the DHCP server you just need to build your scope and activate it.

So on the layer 3 device that is handling the default gateway function for the new network is where you will need to configure the dhcp relay.  

So for instance it might look like the following:

If on layer 3 switch:

vlan 2
  name Network2

interface vlan 2
  ip address 192.168.2.1 255.255.255.0
  ip helper-address 192.168.1.10

The address defined as the ip helper-address is the address of your dhcp server.

That is pretty much all you need to do.
Damian GardnerIT Admin

Author

Commented:
Ken - thanks for your help on this.  So - when you say the DHCP relay packets will identify which network it came from - does that mean I will need to specify the ports on my switch stack as either VLAN 1 or VLAN2, and based on which port the clients are connected to, will determine which network they're from?  And do I understand it correctly that I can have both subnets hosted on the same DHCP server?  or do I need a 2nd server to live on the new 192.168.2.x subnet?
Network Consultant
Commented:
Ok so to do this right, you will need to have the users on the 192.168.2.x network to be connected to switch ports that are on vlan 2.  So yes you will need to identify those users that should be on vlan 2 and configure their switch ports to be access ports on vlan 2.  Both subnets can be handled on the single DHCP server.  The DHCP server is currently on vlan 1 which is 192.168.1.x network.  The DHCP server will respond with a 192.168.1.x lease assignment to those users as it will receive broadcast DHCP requests from those users on vlan1.  The same DHCP server will also received DHCP relay packets from the layer 3 gateway device (switch or router) form the users on VLAN 2.  The relay packet will identify that the request is coming from the 192.168.2.x network so the DHCP server will respond with a lease assignment on the 192.168.2.x network.
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Damian GardnerIT Admin

Author

Commented:
ok - that makes sense.  so I need to separate a grouping of ports on the switch and make those VLAN2, it sounds like.  Great - thanks for your help.
Damian GardnerIT Admin

Author

Commented:
One thing I just thought of - what happens if a client is connected thru a VLAN1 port, and the 1st subnet is maxed out and has 0 addresses available?  Will the DHCP server default over to the 2nd subnet and assign a free address from the 192.168.2.x subnet?  Or would the client not get an IP?

Thanks again
Ken BooneNetwork Consultant

Commented:
The client will not get an IP address in that situation.  You will need to change their port to vlan 2.
Damian GardnerIT Admin

Author

Commented:
ok.  I sort of figured that.  thanks again Ken

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