A client of mine is part of a bigger group of companies running a MPLS for connectivity.

My Clients IP Address range is We use this with DHCP and MASK of and our addresses are running out quickly so I need to increase the capacity.

This leaves me with one of two options:

  1. Change the subnet mask to and add another subnet to the dhcp. This will allow me to use 192.168.12/22 or to

  2. Add an IP on the router on and do routing between the subnets on the router.

Problem with option 1 is that is used by another company on the MPLS

Problem with option 2 is that all the traffic between the subnets and will need to go though the router on a 100mbps port? Is this assumption correct or will the devices be able to communicate "directly" without going through the router once the route is established?

I would love to just get a new /22 range as one is available, but all the work involved moving the IP addresses of servers, odbc's ERP system clients etc would be costly.

What would you do?

  • A router or L3 switch is always required to cross a subnet.
    – HAL
    Aug 6, 2014 at 13:55
  • Do you not have access to a L3 switch? Is there a budget or not budget? Aug 6, 2014 at 14:49
  • No L3 switches currently installed, and budget only available next year.
    – user7844
    Aug 7, 2014 at 6:17
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Jan 5, 2021 at 23:11

4 Answers 4


Based on the information you provide, I would go with option #3.

  1. Add a secondary /22 or /23 IP range to the interface.
  2. Test the new range for expected results (especially since some servers/services may limit access by IP in firewall or ACL).
  3. Set this new range as the DHCP scope so clients will move automatically as they renew their DHCP lease.
  4. For devices with static IP addresses:
    • Leave devices with static IPs in the current IP range if it is too difficult to move.
    • Migrate those you can move at your leisure.
    • New static assignments should be put in the new range, so over time the old range will get phased out (even if it takes years).

Obviously there could be a lot more to this plan, but with the limited details provided, this is the framework I would follow.

  • Thanks for your idea. My plan also included this, but my problem is that static addresses are mostly the servers and if they stay on the old subnet, all the traffic to all the servers has to pass through the router with a 100mbps port.
    – user7844
    Aug 7, 2014 at 9:12
  • Generally, most servers are not utilized to a large degree traffic wise. Move those that do have high traffic needs first. Unless the company is willing to spend some money (either in new hardware or time/effort to move devices), they will have to live with the bottleneck.
    – YLearn
    Aug 7, 2014 at 13:27

assuming that you can not buy a new l3 switch, I would go for option 2 (two subnets), but I would make them two separate networks also physically/logically. so if you have several floors on your office, give one subnet for each floor, or give one subnet for servers and one for PC, or at the very least make sure that heavy traffic servers are on the "correct" network.. e.g. that the backup server is on the same subnet as the hosts it will be in charge off. (to avoid overloading your 100Mbps bottleneck.

if you go for the server vs other split, you could also setup good rules in your firewall, to protect your normal PCs better than servers for example.


When the customer is coming into the network, are they utilizing a sub interface or do they have their own at each end? or is all of this being done via the global routing table? I ask because you mentioned MPLS, you could set the customer up to have their own VRF with entry points at each end, for example:

  1. Same physical link, no sub-ints. Place route-map that associates traffic from the customer's address to a specific VRF
  2. Sub-ints, place ip vrf forwarding entry on the sub-int

This would allow you to place the overlapping route in your router.

  • 1
    It's better to ask questions by posting comments under the original question. Then you can later post an answer which takes all of the information into account. Aug 6, 2014 at 18:00

The best solution is to add additional subnets as needed and route between them. Don't increase the size of your broadcast domain, as this will lead to heartburn later. In other words, option 2 is the better choice. And to answer your question, yes, you will need to route between the subnets. In this case, it would be best to use a layer 3 switch to handle your inter-VLAN routing, instead of hairpinning through an external router connected at 100 Mbps.

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