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What is a “subnet mask”?

by Leo A. Notenboom

A subnet mask is a way of telling your computer or router what network addresses are local and which are remote.

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I keep seeing the term “subnet mask” when I configure network stuff. What is that?

Well, to be blunt, it’s something you probably never need to know about. Sure, you may have to enter one into a router configuration, but it’s typically something you’ll be given without needing to know exactly what it means.

You want to know anyway, don’t you? Fair enough.

A subnet mask is just a nifty way to define sub-networks. Besides being completely unhelpful, that definition actually opens up a slightly larger can of worms.

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Phone numbers

I’m going to use phone numbers as an analogy. It’s an imperfect comparison, but I think it’ll help make the point.

In North America, phone numbers are 10 digits long, and consist of three parts 1:

  • A three-digit area code. Originally, an area code defined exactly that – a geographical area. Area code 206, for example, was, at one time, all of western Washington state.
  • A three-digit exchange. The exchange identified the switching equipment that covered a sub-region within the area code. For example, 788 represents the “Duvall” exchange. Located near the city of Duvall in western Washington is a small utility building that houses the equipment that is the 788 exchange.
  • And finally, the four-digit line number. It’s this number that identifies each pair of wires that leave the exchange and arrive at a real telephone set in someone’s home.

So: what we take for granted as a 10-digit “phone number” is really a construction of three distinct numbers, each with a specific meaning.

Internet addresses

Now let’s look at the internet.

You already know that each computer connected to the internet has its own equivalent of a phone number, called its Internet Protocol, or IP “address”. Names, like “askleo.com”, actually map to these numeric addresses, such as “50.28.23.175″.

Subnet MaskAs you might already be thinking, each IP address breaks down into components not unlike the 10-digit phone number. The difference is that the components are not always the same size, which is where the subnet mask comes in.

Each network administrator assigned a range of IP addresses is free to create subnets to divide things within that range and to define how large they are.

It’s the subnet mask that defines how big a part of the internet address is to be used as the subnet number.

Unfortunately, this is also where we have to start thinking like computers, meaning we need to think in binary.

Basic subnet mask examples

We’ll use these addresses:

1) 164.109.28.3   [binary: 10100100 01101101 00011100 00000011]
2) 164.109.27.233 [binary: 10100100 01101101 00011011 11101001]
3) 164.109.139.4  [binary: 10100100 01101101 10001011 00000100]
4) 50.28.23.175   [binary: 00110010 00011100 00010111 10101111]

A subnet mask is a binary number (usually expressed in decimal, like an IP address) whose digits are set to 1 to indicate the positions of an internet address that should be “paid attention to” as the subnet. Conversely, it’s set to zero for that portion of the address that defines the specific computer on that subnet.

For example. let’s look at this subnet mask:

255.0.0.0

In binary, that’s:

11111111 00000000 00000000 00000000

Now compare that with the four IP addresses I listed above, and only pay attention to that portion of the IP address where the subnet mask is a 1.

You’ll notice that the subnet mask is only set to one for the binary digits that correspond to the first number of the IP address. With this subnet mask, the IP addresses that begin with “164” are all on the same subnet, and the one beginning with 50 is not.

That could mean that a router would route traffic for all the 164. addresses one way and the traffic for the 50. address another – all by paying attention only to what the subnet mask indicated was significant.

Advanced subnet mask example

As internet traffic approaches its destination, it’s common to use subnet masks to further refine how things are divied up on the network.

Let’s look at this subnet mask:

255.255.128.0

or in binary:

11111111 11111111 10000000 00000000

The ones in the first two groups of eight binary digits are all set, which means that each of the first two numbers in a decimal IP address would be “paid attention to”. But that third group of eight only has a single one. That means only a part of the third number is significant.

Let’s look at our first three example IP addresses again:

1) 164.109.28.3   [binary: 10100100 01101101 00011100 00000011]
2) 164.109.27.233 [binary: 10100100 01101101 00011011 11101001]
3) 164.109.139.4  [binary: 10100100 01101101 10001011 00000100]

I’ve highlighted the portions of the binary addresses that this subnet mask tells us we need to “pay attention to”. With that highlighting, it’s easy to see that the highlighted portions of the first two IP addresses are identical – they’re on the same subnet. The third is different, albeit only in that last binary digit. That’s enough – it’s in a different subnet than the  first two.

Your subnet mask

Chances are, if you’re using a traditional consumer router of some sort, you have a very simple network configuration and subnet mask.

If you’ve ever looked at your IP configuration by running “ipconfig” in Windows Command Prompt, it probably included information like this:

IPv4 Address. . . . . . . . . . . : 192.168.0.83
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.0.1

If we apply the rules we’ve talked about so far, the subnet mask used here – 255.255.255.0 – tells us a couple of interesting things:

  • All IP addresses encountered on this network that begin with “192.168.0.” are on the same subnet.
  • Thus, all devices with IP addresses beginning with “192.168.0.” are on our local network. The router doesn’t need to touch the internet at all when dealing with IP addresses like this.
  • Conversely, IP addresses that do not begin with “192.168.0.” are not in the same subnet, are not on our local network, and the router must reach out to its external connection – the internet – to send data destined for them.

The “Default Gateway”? That’s the IP address of the router itself on our local network, also within the same subnet.

Subnet masks: why care?

All this brings us to an even more relevant question: why should we care?

Normally, you don’t. There’s little, if any, reason for the average consumer to know about subnets, subnet masks, and routing. Networking is more than complex enough as it is. If you ever need it, you’ll either be told what network mask to type in to some configuration, it will default to the right thing, or a network engineer or technician will set it up for you.

With that pressure out of the way, subnets and subnet masks are all about routing – specifically, making sure that the information you send from your computer makes it to the intended destination computer, and vice versa.

When information flows across the internet, it flows through equipment called routers. Routers look at the IP address the information is destined for, and decide the best way to get it there. A subnet is a quick way to know where the information belongs. For example, a packet from our example #1 to example #2 is on the same subnet, so a router can take advantage of that information, know that it’s “local”, and not send the packet anywhere else. A packet from #1 to our example #3, however, is probably destined for a computer on a different subnet, and the router handling that packet will know to send it along a different path to get there.

To use our telephone analogy again, if I’m in Duvall, making a call on my 788-xxxx telephone to another 788-xxxx telephone, then the phone equipment knows that it doesn’t have to try anywhere else – it all happens within the Duvall exchange. On the other hand, if I try to call a 483-xxxx number, then the 788 exchange needs to route my call to other equipment within my area code that knows how to find the 483 exchange.

As you can imagine, I’ve only scratched the surface here. Networking and the internet are incredibly complex.

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Footnotes & references

1: With changes in technology and phone number portability, much of this is no longer true, of course. But the old system remains useful for comparison.

72 comments on “What is a “subnet mask”?”

  1. Best explanation I’ve ever seen on the subject!
    I just now went from “what the heck is that all about, let’s just use 255.255.255.0” to “AHA.. Now I get it !!” (but will for now still use 255.255.255.0 🙂 )…
    Thank you so much for clearing this up 😉

  5. according to the tech guy at verizon my “ppp subnet mask’ has been changed to all 255’s by what he refered to as a fragment of some sort of spyware, now my computer apparently tries to log onto itself and after a few minutes my internet just crawls because of the lag. is there any way i can change that subnet mask back to another figure, the other two subnets are set to 255.255.0.0.
    any help would be appreciated

  9. Hi. I am supposedly on a two computer network in my home, but have a subnet mask address. Does this mean there is at least one other computer involved? I ask because I believe that there is another one involved in a situation where my internet activities are actually being monitored from a computer outside my home.

    Your explanation was the only one of the ones on the internet that I could understand. Thank you for using the analogy.

  10. There’s always a subnet mask, so seeing one doesn’t really tell you anyting about the presence of any other computers on the network.

  11. This may be the wrong thread for this, but….

    This is the analogy I use for people when I try and explain ip addys… Lets use the default ip for home networks, 192.168.1.1, and actual street addresses as an example.

    192 = the country you live in
    168 = the state you live in
    1 = the city you live in
    the second 1 = your actual street address

    If you have an address of 192.168.1.1 and you send a letter to your neighbor, they would (theroiectily) have an ip address of 192.168.1.2
    When the mail man picks up your letter to deliver it (acting like a router), he would look at the ip addy and see that the address is on the same street, so there would be no need to take it back to the office and send it to the recipient… He could simply walk next door and drop it off himself and it would get there very fast.

    If you were sending a letter to the next city over, but they have the same street address as you… say 192.169.1.1 the mail man (router) would know to bring the letter back to the office and “route” it from there to the next city over and in turn, to the actual street address.

    So on and so on up the line for country (192) and state (168).

    That’s the best way I’ve found to explain it. Bringing dynamic ip’s (mobile homes) into the picture makes things a little more difficult.

  12. hi i’m just getting into subnet masking, but i still don’t get it, is there any where which explain’s this in a more simple way
    thanks

  13. We have a dsl service that splits in our home and my husband uses one and myself the other on my computer for internet access. When I click on the icon it shows me surrounding area wireless users. Some with Secure Access and others with Unsecure access. In a discussion with neighbors, there is a concern that someone can get access into the computer by somehow overriding the subnet mask (router #’s). Is that even possible and should we be concerned or take any precautions to prevent it?

  14. They don’t even have to play with the subnet mask. An unsecured wireless network can be used by anyone close enough to get a signal.

    In cases like that, I strongly suggest enabling WEP, or “Wired Equivalent Privacy” on your wireless networks. It takes a little doing, and education.

  15. hi mike. if what you said is true you live very close to me because my ip address is 192.168.0. not saying the rest. and my friend had 192.168.1. not telling rest but we live in the same town. i dont think that the third number means city.

  16. 192.168 mean that you’re behind a router, and could be anywhere on the planet.

    Mike was just using it as an example, he didn’t mean specificly that IP addresses implied physical location to that degree.

  20. That tool will not tell you what your subnet should be. It’s simply a calculator that assists in determining the values to use. You still need to know, and enter, the details of your network configuration by hand … including what the subnet characteristics are.

  21. i can prevent users accessing my computer using firewall settings for IP address and a subnet mask.
    does that mean i can use the subnet mask to limit access to certain geographic areas ?

  22. dear sir how can i calculat the subnet mask and what would be the subnet mask of the following Destination Addresses
    182.220.10.4
    121.10.3.4
    140.10.34.30
    164.9.2.1
    220.64.32.4
    92.1.3.8

    waiting for your positive response.
    best regards

  23. Hi sir it is very nice but u can explen it. very esy way plz. how to we caluculet very esy way plz explen sir.

    tanks

    somu

  24. Hi,
    I need to find my IP-adress, Subnet Mask, Gateway, Primary DNS and Secondary DNS of my router to make a wireless connection. can you help me find em?

  25. To find your IP-adress, Subnet Mask, Gateway, Primary DNS and Secondary DNS, do the following
    click START -> RUN -> (type cmd in the text box on the RUN, click OK!) -> ipconfig /all (type it in the command prompt!)-> you got what you wanted!

  31. Leo, how will you determine the number of possible subnet masks available and the number of possible hosts available on the subnetwork?

  32. How do i easily convert binary numbers to decimal, vice versa. I can do with my calculator, but have no clue how to manually. I googled it, but I just can’t understand it.

  33. Subnet is broken down into 4 parts called octets (255.255.255.255) each 255 is an octet. Each octet is broken down into 8 bits which are either turned on (1) or off (0). Here is where 255 comes from: (8 bits) 1 1 1 1 1 1 1 1 <– 8 bits all turned on, still following? The bits break down like this 128 64 32 16 8 4 2 1 <– breakdown of what the bits stand for, still following? All bits turned on = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255 If the subnet mask was 255.255.255.248 the bits would look like this 11111111.11111111.11111111.11111000 (128 + 64 + 32 + 16 + 8 = 248) you don’t need the last three bits so they are turned off. Hope this helps, I’m not a teacher but this is the best I could do to try to explain it.

  34. I just bought a router and its asking me for my IP address, Subnet mask, Gateway, and DNS1 and DNS2. Could you help me find those?

  35. Normally that’s all information the router will get automatically from your ISP, OR your ISP will have to provide to you.

  37. —–BEGIN PGP SIGNED MESSAGE—–
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    Leo
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  38. im compltely and thourly confused how do u get from having the 3 ip adress and the subnet mask to tell that their on the same subnet?
    is it just b/c the the first number in the 3rd octet for #1 and 2 is 0 while on #it starts w/ 1?

    Basicly what im asking is u said “If we do the masking and converting and other what-have-you, ” at one part. could u show what the “converting and other what-have-you,” b/c i basily have to be able to do a few wxample problems like this for my class and i have no clue what u did

  39. I think it’s about using AND logic operator on every bit. 1*1=1, 0*1 = 0, 1*0=0, 0*0=0. Apply this on both ip#1 and ip#3 and see that results are different. In other words, the bits on 1 in the subnet mask says where the received packet address should be the same with the local address.

  44. where can i find the sub mask, gateway, the dns1 and dns2 at please help

    Depends on the situation. It’s normally provided by whomever provides the network you are attempting to connect to.

    – Leo
    11-Oct-2008

  45. im very confused with your explanation of subnet masks if i provide you my ip address could you please work out my subnet mask 78.145.35.16. thank you very much.

    I can’t. That’s information that has to come from whomever provides your network connection.

    – Leo
    14-Oct-2008

  46. Thank you for this! I’m in CCNA, and I’ve known how to subnet, and how to do the number crunching, but I just couldn’t figure out what the mask actually DID…Pretty bad, I know. Awesome tutorial though, pretty sure I understand now

  48. 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
    PLEASE HELP!!!
    [email address removed]

  49. “The day you stop learning is the day you start becoming obsolete.”

    Thank you Leo for taking the time to shed some Light on these,[Quote]…translation; the internet’s a complicated world. [/Quote]
    So true.
    And I also thank you for the links U provided 4 further reading on,,,

    A+
    Luc from Montreal, Qc, Canada

  51. Onbard a boat, I have multiple computers connected to the Lan side of a Router (192.168.133.241/ 255.255.255.240 DHCP on 242-250) and a wireless Bridge (DHCP off) connected to the WLAN side of the Router (DHCP client from whoever the Bridge connects with on land). I want the computers on the Lan side of the Router to have access to each other, to the internet (through the WAN port to the Bridge) and (HERE IS THE PROBLEM) to the BRIDGE itself (currently 192.168.133.192 on the router side. Except for the last step all of this works properly.Please help

  53. how do i find my subnet mask number. i’m trying to connect my sons x-box onto my router and i need my subnet mask number and gateway
    PLEASE HELP!!!

  54. Linda (“how do i find my subnet mask number”):

    If using Windows VISTA just click once on the little circle in the lower left corner.
    Then in ‘start search’ type RUN.
    It will prompt “open” type CMD.
    That will prompt a DOS window.
    Type: IPCONFIG
    Press Enter and all your info will appear.

    In Windows XP
    just click once on the little START button in the lower left corner.
    Then click on RUN.
    It will prompt a DOS window.
    Type: IPCONFIG
    Press Enter and all your info will appear.

    Good Luck

  56. A small detail:
    With a subnet mask of 255.255.255.0 any IP address beginning with 192.168.0 is an address that is defined as being in private address space.
    That is why it is part of our local network. Addresses in private address space will never be assigned to any organization.

    • That’s actually independent of the subnet mask. IP address ranges beginning with 192.168, as well as the 10.x.x.x and the 172.16.0.0 – 172.31.255.255 range are all defined as private/local and would never appear on the open Internet.

  57. It seems to me that with a subnet mask of 255.0.0.0, 192.168.x.x is not a local address but a good IP address.
    The 192 being a good Network address, and the 168.x.x being the host address.
    Am I looking at this wrong?

    • I’m not really sure what you’re saying. 192.168.x.x is, always, a private IP address you would never see in the internet. 192.168.x.x with a subet mask of 255.0.0.0 simply means that anything starting with 192 is local to the device configured with that mask (or included in whatever that device is configured for).

  59. Leo, I have worked in computing since 1967 and that is the BEST (and the MOST understandable) explanation of subnet masks I have ever seen.
    BRILLIANT! (as usual) – THANK YOU!

  60. The mask divides the IP address into the network portion and what is left over for host addressing.
    It does this by all using those numbers which tends to cause confusion.

  61. Hi Leo, thanks for making so much of this so easy to grasp! The part that confused me was where you gave the example of 255.255.128.9 and wrote, “The ones in the first two groups of eight binary digits are all set, which means that each of the first two numbers in a decimal IP address would be “paid attention to”. But that third group of eight only has a single one. That means only a part of the third number is significant.” Would that type of subnet mask ever exist? I have only ever seen where 255 & O were used for the octets… THANKS!!

    • First off that’s 255.255.128.0, not 9. But absolutely. In fact ANY value can be used in some very bizarre networking scenarios. Very commonly, though, (and thinking in binary) the values that represents some number of 1’s all set beginning at the left are very common. 128 in binary is 10000000.

  62. So Leo -is it possible for someone to change something in the subnet mask that allows them to receive all your e-mails ( you still receive them too) to spy on you or like a hack to get important information about you. History on spouses phone show attempts to alter the subnet mask, is this him attempting to change this or outside hack attempt?

  63. Leo, a simply magnificent explanation of the subnet process using IPv4. How about an update on the subnet process using IPv6 since they are close to running out of IPv4 addresses. Lots more to play with there.

  64. Hey Leo, thanks for the article!

    What happens when I need to communicate to an IP address outside of my network that shares an IP with something on my LAN? So, say my printer is 10.49.2.13 but a website I want to access is also 10.49.2.13. Does this kind of thing happen?

    Thanks!

    • That couldn’t happen. IP addresses outside of a local area network (LAN) are unique. The IP address of a device in a LAN can’t have the same IP number as a site on the internet. There is a set of internal IP numbers reserved for use within a LAN. For example, your printer may have the same internal IP address a somebody’s computer within their LANk, but those would be invisible outside their LANs and your computer could only see the IP address of the router controlling that LAN.

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