Error conflicted subnet addresses

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An IP (or Internet Protocol) address conflict occurs when two different systems on the same subnet are assigned or otherwise have their network interface configured to use the same IP address. It is possible to have two systems on different subnets configured with the same IP address, but not have a conflict occur due to routing limitations or because of the effects of NAT or Network Address Translation. Systems with the same IP Address on the same subnet need not have the same netmask or broadcast address for the conflict to occur.

Consequences of IP Address Conflicts

Systems handle IP address conflicts in multiple ways. In the past, many systems would simply act as if they were not in conflict, and connectivity to both or all systems would be intermittent and unreliable. In most modern operating systems, the second system to come online with the same IP address on the same subnet as another system would just disable their network interface and give an error until the situation is resolved by the administrator. This allows the original system to continue functioning without disruption, but completely breaks IP base communications for the second system. Of course, in production and critical environments, this result is preferred.

How IP Address Conflicts Cause Problems

IP address conflicts do not have a major impact on layer 3 or layer 7 communications (where most tcp/ip communications take place), but on the lower layer 2 protocols that layer 3 and above depend on. Once packets arrive to their destination network, or the subnet that the target system resides on, an ARP request is generated asking which system has the target IP address. If a routing or switching device that stores ARP information has this information cached in its ARP cache, that data is automatically used for decision making. If not, the first system responding to the request stating that it has the target IP address is added to the cache, and then packets are then forwarded to that host using its MAC (Media Access Control) address. Depending on system and network states or utilization, different systems may win the ARP response race, causing packets to be sent to one system at one time, and the other system other times. For IP communications this kind of reliability issue is unacceptable.

Resolving an IP Address Conflict

Resolving an IP address conflict is simple, merely assign one of the hosts a new, and not currently used IP address. Changing system IP addresses can have other consequences. It is common for commercial software licensing to be tied to a specific IP address. Other times, applications hard code IP addresses in their configuration files, causing application communication problems after an IP address change. Any time you change a system’s IP address, it is best to test all applications for due diligence.

Avoiding an IP Address Conflict

It is prudent to use some IP address assignment tracking method to determine which IP addresses have been assigned and are active. Spreadsheets, such as Microsoft Excel, are often used to track a small number of IP address assignments such as for a standard class C subnet (254 IP addresses or fewer). If you must track more than a single subnet, or larger subnets, it’s in your best interest to use a database application, or custom commercial software packages to assist in the tracking of information. In many cases, commercial packages for IP address management can be configured to also provide DNS management services as well. Understand your personal and organizational needs, and pick a solution that suits your situation.

Even in cases when management softwar is used to track IP address assignment, it is not uncommon for IP addresses to be assigned by employees, contractors and other personnel who do not have access to or concern for the IP address management solution. Before assigning an IP address to a system, always check to see if a system responds to that IP address by pinging it from a system with communication to the subnet the system would reside on. If a response to the ping is seen, assign a different IP address or resolve the conflict before placing the new system on the network.

Yes, and the good lesson here is to never ever use 192.168.0.0/24, 192.168.0.1/24 in corporate environment, just like never to use 802.1q VLAN ID 1.

But, there are cases when you can’t do anything. For those cases, OpenVPN starting from 2.4 supports internal stateless address translation (NAT), a.k.a. network mapping. From man openvpn:

--client-nat snat|dnat network netmask alias
        This pushable client option sets up a stateless one-to-one NAT rule on
        packet addresses (not ports), and is useful in cases where routes or
        ifconfig settings pushed to the client would create an IP numbering
        conflict.

        network/netmask (for example 192.168.0.0/255.255.0.0) defines the
        local view of a resource from the client perspective, while
        alias/netmask (for example 10.64.0.0/255.255.0.0) defines the remote
        view from the server perspective.

        Use snat (source NAT) for resources owned by the client and dnat
        (destination NAT) for remote resources.

        Set --verb 6 for debugging info showing the transformation of
        src/dest addresses in packets.

So, you add a line into your client .conf file:

client-nat dnat 192.168.2.0 255.255.255.0 192.168.1.0
route 192.168.2.0 255.255.255.0

and then if your server-side has 192.168.1.5 system, the client will be able to access it as 192.168.2.5, resolving an addressing conflict. And so on.

---

@VinceCz wrote:

In fact I have a ZTE modem which is connected to a hub, this hub goes to wired network in each room, …  all devices which are connected directly (wired) to the ZTE is in 10.0.0.xxx

---

As this all works, the ZTE is a router/modem.

---

@VinceCz wrote:

I want same IP mask before and after the router (Nighthawk R7000).  

---

The R7000 in router mode is isolating, NATing, and firewalling — just like a NAT router is supposed to do. A router is to be used between the Internet/WAN port and the LAN. You can’t. 

---

@VinceCz wrote:

…, including the R7000 + some others devices, printer, mycloud, tv, DNLA. I am facing lot of device sharing issues (one laptop does not see the printer, DLNA does not find mycloud, the tv cannot work with Internet) … , and all which is in WIFI is in 192.168.1.xxx. Hence trying to have all in 10.0.0.xxx even wifi devices, but not able to do the change. Also so far I didn’t do any tricky change 

---

Issues predictable — you have two networks, to LANs. Even if you manage to configure the same IP subnet on both sides (the router Web UI is prohibiting for good reasons): The router is still a router and can’t work because it does not know which network you want to reach with the same addresses on both sides.

Correct config is always WAN/Internet [e.g. true Modem] <-> Router WAN/Internet Port <-> all the LAN

What is the plan, for what purpose have you bought the R7000 exactly? If the idea is just offering wireless, you might re-configure the R7000 as a plain wireless access point — check the documentation. Be aware that only one Ethernet port (typically the WAN/Internet port)  might be workable for connecting the local network, all others don’t work or might be ridiculously slow. Be aware agin that consumer routers make poor wireless access points.

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I have a CIDR block of 172.30.0.0/21 and need to create 2 private subnets and then 2 public subnets for it.

At first I created:
for private: [«172.30.8.0/21», «172.30.16.0/21»]
for public: [«172.30.0.0/22», «172.30.8.0/22»]

but then I got the following errors:

Error: Error creating subnet: InvalidSubnet.Range: The CIDR '172.30.8.0/22' is invalid.
10:59:12    status code: 400, request id: 9***
10:59:12  
10:59:12    on .terraform/modules/vpc-us-west-2/main.tf line 335, in resource "aws_subnet" "public":
10:59:12   335: resource "aws_subnet" "public" {
10:59:12  
10:59:12  
10:59:12  
10:59:12  Error: Error creating subnet: InvalidSubnet.Range: The CIDR '172.30.16.0/21' is invalid.
10:59:12    status code: 400, request id: d**
10:59:12  
10:59:12    on .terraform/modules/vpc-us-west-2/main.tf line 363, in resource "aws_subnet" "private":
10:59:12   363: resource "aws_subnet" "private" {
10:59:12  
10:59:12  
10:59:12  
10:59:12  Error: Error creating subnet: InvalidSubnet.Range: The CIDR '172.30.8.0/21' is invalid.
10:59:12    status code: 400, request id: d**
10:59:12  
10:59:12    on .terraform/modules/vpc-us-west-2/main.tf line 363, in resource "aws_subnet" "private":
10:59:12   363: resource "aws_subnet" "private" {

So NOW:-

• I created:

for private:[«172.30.0.0/21», «172.30.4.0/21»]
for public: [«172.30.0.0/22», «172.30.2.0/22»]

are they fine? will they work and fix the errors?

• This is part of my code:

# List of private subnets to create in the environment, e.g. ["172.18.0.0/21", "172.18.8.0/21"]
variable "private_subnets-west-2" {
  type    = list(string)
  default = ["172.30.0.0/22", "172.30.6.0/23"]
}

# List of public subnets to create in the environment, e.g. ["172.18.168.0/22", "172.18.172.0/22"]
variable "public_subnets-west-2" {
  type    = list(string)
  default = ["172.30.0.0/22", "172.30.4.0/23"]
}

(the values currently in the subnets, I am not sure they work)

and these are the subnets I need to create. Again my CIDR is 172.30.0.0/21

Update: my private subnets work fine:

# List of private subnets to create in the environment, e.g. ["172.18.0.0/21", "172.18.8.0/21"]
variable "private_subnets-west-2" {
  type    = list(string)
  default = ["172.30.0.0/22", "172.30.6.0/23"]
}

However, I have been trying different subnets for my public one’s since an hour now, and cannot find the proper ones:

# List of public subnets to create in the environment, e.g. ["172.18.168.0/22", "172.18.172.0/22"]
variable "public_subnets-west-2" {
  type    = list(string)
  default = ["172.30.0.64/26", "172.30.0.128/26"]
}

keep on getting errors for conflict or invalid:

Error: Error creating subnet: InvalidSubnet.Conflict: The CIDR '172.30.0.128/26' conflicts with another subnet
        status code: 400, request id:

  on .terraform/modules/vpc-us-west-2/main.tf line 335, in resource "aws_subnet" "public":
 335: resource "aws_subnet" "public" {

Error: Error creating subnet: InvalidSubnet.Conflict: The CIDR '172.30.0.64/26' conflicts with another subnet
        status code: 400, request id: 

  on .terraform/modules/vpc-us-west-2/main.tf line 335, in resource "aws_subnet" "public":
 335: resource "aws_subnet" "public" {

I have been stuck for long, if someone can help me and tell me exactly which 2 subnets I can use, it would be great help!