In making network changes how would mixing sub nets affect the network

Question

Will hosts with IP address manually assigned as 192.168.0.10/24 or 192.168.1.11/24 continue to function. When the router, DHCP, and switches are now setup to work as 192.168.0.0/22.

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

Simply we have hosts and users in the following networks

192.168.0.0/24 GW 192.168.0.1
192.168.1.0/24 GW 192.168.1.1 - host/Firewall 192.168.0.100
192.168.2.0/24 GW 192.168.2.1 - host/Firewall 192.168.0.100

Can I create super net / larger sub net network like 192.168.0.0/22.

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 192.168.0.10/24 or 192.168.1.11/24 when the router, DHCP, switches are now setup to work as 192.168.0.0/22 continue to function.

Answer 1

Will hosts with IP address manually assigned as 192.168.0.10/24 or 192.168.1.11/24 continue to function. When the router, DHCP, and switches are now setup to work as 192.168.0.0/22.

Not really. The hosts will continue to talk to each other BUT moving a gateway/router out of their 192.168.0.0/24 makes them inaccessible even though from the router's perspective everything is all right. For instance, a gateway with IP 192.168.1.1/22 cannot be used by a host with IP 192.168.0.10/24.

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: 192.168.0.10/24 and B: 192.168.1.10/22 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 192.168.0.0/24). But if you've got a (hairpin) gateway for A with L3 interfaces 192.168.0.1/24 and 192.168.1.1/24, that route is covered as well.

Answer 2

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

• Three networks just like yours 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24, 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: 192.168.1.1 192.168.2.1 and 192.168.3.1
• 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.