Creating new subnet/DHCP range.

Posted on 2009-04-28
Last Modified: 2012-08-14
I have been tasked with creating another subnet with a range of DHCP addresses.

Currently we have all servers and workstations on  Servers/Printers are configured with a static IP and reservations for them are on the DHCP server.  Workstations pull DHCP.

We are running out of IP addresses, so we want to create the subnet.  Making this subnet the one with DHCP running for the workstations and the servers/printers remaining on the '254' subnet with no DHCP.

All hosts are on a stack of HP Procurve Switches that connect to a Cisco ASA to get out to the web.

Besides creating the range on the DHCP server and removing the '254' range from the pool, what else will need to be configured to make these subnets talk to each other?  

Do any routes need to be added to the ASA?    

I feel like I am forgetting something, perhaps how the DHCP server sitting on '254' will be able to provide addresses to workstation on the '253'?  But these will be on the same switch, so not a problem?

Thank you for any help.

Question by:tpearson1
    LVL 2

    Accepted Solution

    If the problem is just running out of IP addresses, then why don't you just change the network mask to It will let you have IPs in range -

    Author Comment

    Would that be as simple as changing the Range on the DHCP server then changing the Subnet Mask on the Statically assigned hosts?

    What other considerations or pitfalls could this cause?  

    LVL 2

    Expert Comment

    It should be as simple as that. That's the idea of network masks.
    The only pitfall I can think of is that your router and DHCP server may be not capable of handling so much IPs.

    Author Comment


    Did you mean

    I apologize, its just been years since I calculated any subnets.

    Thank you.
    LVL 9

    Expert Comment

    I generally dont recommend going below a /24 to keep broadcast at a minimum and also segmenting makes it easier to filter traffic in access-lists or creating routing policies.

    But before you segment you should think about where you are going to route the vlans. Are you going to use a L3 switch (recommended) or are you gonna use the ASA?

    If you use the ASA there are limits to consider like throughput and if you use Wake-On-Lan it wont work with the ASA as router.

    No matter what, the ASA needs to know about all subnets in the internal network that wants to use the internet else it will drop the packets.

    Author Comment

    So if I do not want to drop below a /24 mask AND do not want to use VLAN tagging, what are my options?

    The ProCurve 2650s are L3 switches.  

    Provided I put a route in the ASA to route traffic on the new subnet to the internet, what else is left to do?

    Thank you

    LVL 9

    Assisted Solution

    You still need to make a VLAN for each /24 net you want to make.

    You connect your L2/L3 devices via VLAN Trunks to a central L3 device, that will route the networks internal and you make from that L3 device a network where you attach the ASA, which has all the routes of the internal network pointing back to the L3 device IP address.

    It is not a good practice at all to use more than 1 network on the same L2/VLAN so if you wont do the above your only option is to use a wider netmask and thus creating more broadcast and make your network more susceptible to errors.

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