Ethernet/Networking, Subnet Masking

Hi all-

So I took the Ethernet class at Rockwell Automation, and I still have the workbook to prove it. It touched on assigning subnet masks, a common mask being 255.255.255.0, with 0 being the host. The instructor said if you assign a mask of 255.255.0.0, it will allow you to connect to different subnets. My question is, if I were to assign my laptop a mask of 255.255.0.0, would I still be able to connect to other hardware with their masks still set to 255.255.255.0? Or do all subnets need to be set the exact same? I do realize this is more than likely basic stuff, but the class didn't really cover hard specifics on the matter. It was more of a "how to get a new piece of hardware up and running super fast, and here's the help line number when you don't figure it out" course. I took a few good things from it, but it sparked a lot more questions in the process. Hell, I still can't even get my wifi routers at home to work together to extend my signal! I appreciate any and all insight!

JasonTheSparky said:

Hi all-

So I took the Ethernet class at Rockwell Automation, and I still have the workbook to prove it. It touched on assigning subnet masks, a common mask being 255.255.255.0, with 0 being the host. The instructor said if you assign a mask of 255.255.0.0, it will allow you to connect to different subnets. My question is, if I were to assign my laptop a mask of 255.255.0.0, would I still be able to connect to other hardware with their masks still set to 255.255.255.0? Or do all subnets need to be set the exact same? I do realize this is more than likely basic stuff, but the class didn't really cover hard specifics on the matter. It was more of a "how to get a new piece of hardware up and running super fast, and here's the help line number when you don't figure it out" course. I took a few good things from it, but it sparked a lot more questions in the process. Hell, I still can't even get my wifi routers at home to work together to extend my signal! I appreciate any and all insight!

Click to expand...

Short version: Assume your PC is set to 192.168.1.1/255.255.0.0, and your PLC is set to 192.168.0.1/255.255.255.0. Your PC will send packets to the PLC, and the PLC will receive them. However, the PLC will not be able to respond, unless there is a router to forward the packet. This means that you can get some weird situations where a connection will be partly created and then error out. It is generally considered bad network design to have devices connected to each other with different subnet masks.

background explanation: The subnet mask doesn't affect receiving packets, it only affects sending them. A device will process any packet sent to it (unless it has a firewall, etc). If a device wants to send a packet to an IP address, it will send it as long as it is in the correct subnet. If it is outside that subnet, then the device sends it to its gateway/router, assuming one is defined. If it isn't defined, then the data is dropped. The gateway/router is tasked with knowing the network layout, and where to send packets with different defined subnets. This is why the PLC will receive the packet from the PC, but then not be able to respond. This is true whether the communication is a complex TCP/IP protocol or something as simple as PING.

That. Is an excellent explanation. Thank you.


Any experience in home networking? lol More specifically, adding a second wireless router to work along with the main without flashing firmware?

JasonTheSparky said:

That. Is an excellent explanation. Thank you.


Any experience in home networking? lol More specifically, adding a second wireless router to work along with the main without flashing firmware?

Click to expand...

Depending on the brand, settings may be named different things. I probably can't tell you what settings you need for your device. I might be able to point you in the right direction, though.

I'm assuming you want the 2nd router to extend the area covered by the wifi system?

Two ways to do it:
1. Run an ethernet cable from one router to the other, and give them the same SSID. The 2nd router must operate as a bridge/wireless switch. There may be a setting for this (turning off routing/NAT), or you may just need to simply plug the 1st router into one of the "local" ports instead of the "internet" port.

Pro: fastest network performance, most reliable
Con: have to run an ethernet cable, which sometimes defeats the point of wifi



2.You can also set up something called a wireless bridge/universal repeater/WDS. Your second router may be able to detect the settings of the first one, and automatically forward the wireless traffic, or it may need to be configured.


Pro: No ethernet cable required, setup is simply changing settings
Con: Requires the 2nd router to be in range of the first router, with a good strong signal. This means it probably needs to be placed a lot closer to the first one than you might hope. Also, slows the communication, because wifi is a shared medium. If a device connected to ROuter two wants to send data, it goes from the client to router 2, then router 2 to router 1. Router 1 cannot talk to any of its clients while this is going on, because it would cause interference.


I would reccommend option 1, if you can. The first rule of wireless is "use an Ethernet cable whenever possible". Especially for infrastructure, and things that don't move, it is so much easier to buy a 100ft ethernet cable (from the internet, where it'll cost about 1/10th of best buy) and run it across your attic than it is to hope the wifi doesn't **** out when your microwave runs.