Will hosts with IP address manually assigned as or continue to function. When the router, DHCP, and switches are now setup to work as

We in inherited an organic network. With three different networks and gateways.

Simply we have hosts and users in the following networks GW GW - host/Firewall GW - host/Firewall

Can I create super net / larger sub net network like

We have a most of the users and some of the hosts allocated IPs via DHCP. We can update this configuration to fix the subnets and Gateway to a single IP.

What would happen to the hosts not on DHCP, which have interfaces and gateways assigned manually.

I know I would need to update the gateways, to get internet access. But how would hosts which have network configuration defined in a smaller limited subnet be affected.

Will hosts with IP address assigned as or when the router, DHCP, switches are now setup to work as continue to function.

  • 2
    Your description of your network can't be right. Your gateway addresses for and are impossible. The gateway address must be in the same subnet as the host.
    – Ron Trunk
    Aug 16, 2019 at 12:06
  • @RonTrunk Hi, sorry you were correct, sorry I missed the complete configuration.
    – nelaaro
    Aug 16, 2019 at 12:15
  • We need more information in order to help you. Please provide a simple diagram and the model of router/switches you have. Note that manually configured devices ignore DHCP protocol, and aren't affected by it. Are all gateways on the same device?
    – Ron Trunk
    Aug 16, 2019 at 12:21
  • 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 can provide and accept your own answer.
    – Ron Maupin
    Dec 15, 2019 at 6:20

2 Answers 2


Will hosts with IP address manually assigned as or continue to function. When the router, DHCP, and switches are now setup to work as

Not really. The hosts will continue to talk to each other BUT moving a gateway/router out of their makes them inaccessible even though from the router's perspective everything is all right. For instance, a gateway with IP cannot be used by a host with IP

All nodes in an IP subnet must agree on their subnet mask - if not, they can't talk to each other without using a router.

With hosts being configured by DHCP, simply change the DHCP scope's network mask and you're good.

With a mix of DHCP and statically configured hosts you need to set up gateways that route between the /24 subnets - even if they're in the very same L2 segment. Leave the gateways' network masks at /24 as long as they're used. That should only be a temporary workaround for a new numbering plan however.

Keep in mind that two nodes can only ever talk to each other when either

  1. they are in the the same subnet, from each node's perspective
  2. there's a gateway in between with an interface in each subnet

Actually, it is possible to use overlapping subnets but it's not for the faint-hearted - and it's definitely not recommended for more than a transition period.

For instance, you've got two nodes A: and B: in the same L2 segment. B talks to A directly since it's within B's subnet.

A can't talk to B directly (B's not part of But if you've got a (hairpin) gateway for A with L3 interfaces and, that route is covered as well.


During a hideous transition period, I had to deal with a very similar problem.

  • Three networks just like yours, and each had .1 as its default gateway.
  • I merged them all into a single ethernet with a single router
  • All new addressing was done in /22
  • Secondary IP addresses were put on the router: and
  • Disabled ICMP Redirect messages on the router

You get asymmetric routing between old /24 and new /22 hosts: old to new will go via the router, while new to old will go direct.

Frankly it's hideous and hunting down and removing the old configurations takes much longer than you'd believe. But it does work.

  • In order for hosts to communicate to hosts in the / 22 range / subnet. They have to have the same subnet. So this could still cause issues but might be a useful work around.
    – nelaaro
    Aug 19, 2019 at 5:59

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