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How do I determine the subnet mask I'm supposed to use?

How do I find my subnet mask number. I’m trying to log onto my
router and I need my subnet mask number, gateway, dns 1 and dns 2
numbers.

I’m guessing you’re not actually trying to logon to your router, but
rather configuring your router so that you can use the connection to
the internet provided by your ISP.

Exactly how we do what we do next depends on the specific router, so
I’ll use mine as an example.

But here’s a hint: none of it depends on knowing a subnet.

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As you point out, there are several pieces of information your router needs in order to provide your internet connection:

“… in the vast majority of consumer, home and even small business connections you never have to know any of this.”
  • Your IP address – this uniquely identifies your internet connection, and is actually the IP address of the router on the internet.

  • The subnet mask – a somewhat mysterious number that actually lets your router know what other IP addresses are “nearby”

  • The gateway address – the IP address of the upstream device – the “gateway” to the rest of the network – to which your router should send packets for devices that aren’t “nearby”

  • IP addresses for one or more DNS servers – the servers that map names (like “ask-leo.com”) to IP addresses.

Here’s the trick: in the vast majority of consumer, home and even small business connections you never have to know any of this. It’s all provided automatically.

Router IP Configuration

Routers can be configured to use several different approaches to connecting to your ISP. The most common is called “DHCP”, or what’s referred to by my LinkSys above as “Obtain an IP automatically”. That’s actually a tad misleading, since the router then gets not only the IP address, but all the other information I listed above, automatically, from your ISP.

You don’t have to enter a thing.

Which, of course, is why it’s the most common approach.

The opposite end of the spectrum is the “static” IP. When using a static IP, the IP address you should use, as well as all of the other information, is provided by your ISP. Be it in an email, some web instructions, or over the phone – this information is provided by the ISP so that you can then manually configure your router.

You don’t have to “find it”, you don’t have to “figure it out”, you just enter the information that’s been provided by your ISP, or contact them if you’ve not received it.

The real problem, of course, is that when configuring a static IP to connect to the internet you can’t figure it out on your own. All of these values are assigned and defined by your ISP.

If you find yourself in a position where you think you need to configure a static IP address, check with your ISP for what they expect you to do.

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4 comments on “How do I determine the subnet mask I'm supposed to use?”

  1. This person might be asking something like this:

    How can I find the default gateway (not subnet) of my router if DHCP is disabled on the local end? I can’t be automatically assigned a LAN IP by my router so I have to manually do it. How do I find out the static information from an unknown router with an unknown default gateway? I can keep guessing using my browser and attempt to log in, but is there an easier way?

    In something like this, I would suggest resetting the router which will then reenable DHCP on the local side by default. This will at least give you an automatic connection which you can use to write down the important info (most importantly the “default gateway”, which you can then use to log into your router and get the DNS servers from your ISP).

    The subnet mask simply tells the router what range of IP addresses it will relate to, usually 255.255.255.0

    Reply
  2. Try this.

    From the Start button select Run
    Enter cmd and press enter
    In the window that opens type
    ipconfig /all

    This will provide all the information you are looking for.

    Regards

    Ken

    Nope. That’ll show you what you have once you’re connected, but not what you should be configuring things to in order to connect.

    – Leo
    10-Jun-2009
    Reply
  3. @ leroy Ken, the command screen ‘flashes’ with no chance to read. How to hold the screen on?
    **************

    Type cmd first in the run box, this will open a command line box where you can type ipconfig or ipconfig/all
    jp

    ps plz excuse the 4 year delay … 😉

    Reply

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