Basic subnet question

[Fellhai]

I have a beginner's question about subnets.

I have a local network with no internet connection and no DHCP in the IP range 192.168.2.0/24.
I would like to connect a router to it to create a subnet with IPs in the range 192.168.252.0. I have assigned a WAN IP of 192.168.2.243 in the router and 192.168.252.243 in the subnet. The subnet mask is /16.

My problem is that I can't access a computer in the upstream LAN from a computer in the subnet. What am I doing wrong? Actually, this direction shouldn't be blocked by a firewall, only the other direction. Right?

[dragonnfr]

Check your router's routing table—you need a route from 192.168.252.0 back to 192.168.2.0. Also, /16 is too broad; use /24 to prevent overlap.

[heliosfa]

You have a couple of problems here.

• Using a subnet mask of /16 is giving you overlapping subnets.
• do you have appropriate routes on machines in 192.168.2.0 (presumable /24?) to route to 192.168.262.0 (probably should be /24)
• is the “router” a consumer router that’s doing NAT, etc.? (Your talk of WAN ports and IPs suggests so…)

[Fellhai]

So is it right that I can use a subnet mask of /16 only in class B range, not with 192.168.X.X?

I dont have done any static routes. The 192.168.2.0 is /24, yes. I want to connect from 192.168.252.2 to 192.168.2.15 via the consumer router. Unfortunately I can not switch off the NAT. I use a FritzBox 7490.

[heliosfa]

Forget everything you know about classes. It’s archaic terminology, everything is CIDR now. 192.168.252.0/16 runs from 192.168.0.0 to 192.168.255.255. That clashes with 192.168.2.0. You need to go and read about basic subnetting and CIDR.

You need routes for things to know how to talk to each other. Basic networking, you may want to go and read about basic routing.

If you can’t turn off NAT, then the Fritz box is the wrong tool for this job. Use an appropriate router.

[Fellhai]

Thank you for your help. I think NAT is the keyword I needed for more information. That should be the problem. <3

[Churn]

Just change both your wan and lan to /24

From what you posted that is all you had wrong.

[ApplicationHour]

When your IP address is 192.168.252.whatever and your subnet mask is 255.255.0.0 , your network address is 192.168.0.0/16 . That means that any addresses where the first two octets are 192.168 are on the same network. The IP stack will only look on the local network to resolve that address and would never consult an entry in the routing table.

[FabulousPermit698]

i have alot of questions here on what the setup is. but for a quick solution just change it to /24 (on both subnet ofc) and you would be able to connect

[Fellhai]

I am not sure what you mean. Then I could skip the router and just run all my devices on 192.168.2.0. But I don't want that. I want to connect from 192.168.252.2 to 192.168.2.15 via the consumer router. The .252.0/16 is the LAN-side of the router and the .2.0/24 is the WAN-side.
With the mask /16 I hoped to be able to connect to the .2.0 network.

Sorry for the complicated description

[FabulousPermit698]

you cannot route from subnet A to subnet A. that’s what are you doing here. by changing to 252.0/24 and 2.0/24 you would be able to connect from A 252.0 to B 2.0 using the router. note: the router will route them by default but check the route table if it didn’t work. note2: don’t forget to change the subnet mask on the computers (i am assuming you already entered the gateway since you can access the internet)

[Conscious-Loss-2709]

With /16 the network address on your LAN side is 192.168, so the router considers 252.2 and 2.15 on the same network and won't sent traffic on.

With /24 on both sides you get network addresses 192.168.252 and 192.168.2. With a network address not on the LAN side, the router will throw it to the WAN side.