Identify Purpose of Subnetting?

geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□

The Comptia updated Net+ Objectives ask for you to Identify Purpose of Subnetting.
What does this mean? All I know is what subnet goes with what IP. Is this enough or do I need to be able to do subnetting in my head?

Thx in advance.
Ciao and Meow




  • vexvex Member Posts: 113
    Identify Purpose of Subnetting = Know when and why you would need to subnet.

    Thats how it registers in my head.

    You dont really have to be able to do it in your head. The test center usually gives you something to write on, so you should be able to do it on paper.

    Ancient Certs:
    Exam 70-064: Implementing and Supporting Microsoft Windows® 95
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  • geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□
    So when and why would I subnet? To make more hosts on my network?

    I think I've got the major part the binary to decimal piece.

    I've taken this from
    Subnetting, or splitting the hosts part, is possible by defining a subnet mask. To start your way in determining the subnet mask that is most suitable for you, determine the number of required subnets. Possible number of subnets are none, 2, 6, 14, 30, 62, 126 or 254. Convert the number of required subnets to binary. For example:

    30 subnets = 00011110 GOT THIS PART!

    Now, count from right to left the numbers until you reach the last bit set to one. In this case that is five bits. GOT THIS PART!

    ***To form a subnet from this number, apply the number of bits on the subnet mask (?), from left to right. So, that would look like this:

    5 bits from left to right = 11111000 binary = 248 in decimal.

    ??? PLZ HELP!
    Ciao and Meow


  • JuddJudd Member Posts: 132
    Ok, here's an easy way to subnet.

    I'm assuming you understand the classes of subnets...A-B-C

    Let's say you have IP with a default subnet mask of This is 1 network with 256 hosts. All the 255's in the subnet represent network bits, the 0 represents the host bits. This means that you can technically have 256 hosts...2 .3 .4.-.254 .255 .256

    This isn't always practical if you wish to have more than one network so you can subdivide that IP address and get more networks with fewer hosts per network.

    So let's say you want 5 networks from that one IP address of The formula is 2^x -2 to figure this out. So, 2^3 = 8 - 2 = 6. You can have 6 useable networks by subnetting this address. So you are really "borrowing" 3 network bits from the 3rd octet. So by using these 3 bits to make more networks you have to turn them into 1's because they are network bits now. So, 128+64+32=224. Now 224 is your new subnet mask in the 4th octet. 5 bits remain, so 2^5 – 2 leaves you with 30 addressable hosts on each new network.

    IP =
    Subnet =

    How do you know what your 6 new networks are? Easy, subtract 224 from 256 to get your first network address. 256-224=32 Then add 32 to each new number until you reach the subnet mask of 224. So, 32+32=64+32=96+32=128+32=160+32=192+32=224 the subnet mask.

    So here are you new networks:
    192.168.10. -

    Of course, these numbers represent network and broadcast addresses, useable addresses are in between these.

    A little over simplified but hope this helps!
  • WebmasterWebmaster Admin Posts: 10,292 Admin
    @geekgodes: I don't mind the questions at all, but this is the second time you asked something to which you can find the answer in my Network+ TechNotes, so I suggest reading those first. In this case:
    I wrote:
    Calculating the correct subnet masks for specific scenarios is not something you will be tested on for the Network+ exam but it is important to understand what subnetting is.
    What it is is explained in that link above.

    Regardless, subnetting is an important subject when it comes to networking. To get a better and more detailed understanding of what it is and how it is done, try
  • jlewis92683jlewis92683 Member Posts: 8 ■□□□□□□□□□
    here is an explination of subnetting and its purpose. subnetting is used to divide up your network mostly for the sole purpose of broadcast domains and controlling network traffic. standard subnetting does not give you more hosts, you actually loose hosts at a rate of two per subnet one for network and one for broadcast. You shouldn't have to worry much about breaking down subnets unless you are going for cisco certs. hope this helps
  • geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□
    To Webmaster,

    I actually did read the TechNotes, and many other explanations. None of them seemed to answer the 2.7 CompTia Objective of purpose of subnetting.
    Also, there seemed to be an assumption on how you achieve the X portion of 2^x-2.

    Besides, haven't you heard the saying, "The only stupid question is the one that isn't asked."? Also, I thought that is what forums were for... questions.

    All I wanted was a simple and brief explanation, which seemed to be hard for me to find anywhere.

    Thanks for everyone's help.
    Ciao and Meow


  • WebmasterWebmaster Admin Posts: 10,292 Admin
    Really, don't start this negative defensive crap on me today... I clearly stated: "I don't mind the questions at all"

    The purpose of subnetting is explicitely mentioned in those TechNotes:
    This process is known as subnetting. The main reason to divide a Class A, B, or C network into smaller subnets is to use the available address space more efficiently. For example, your company is assigned a Class B network, which allows for 65534 different host addresses. It would be a waste of addresses to use the entire range for a single network with 200 nodes. Instead, the class B address can be subnetted by using Class C subnet mask, or a classless subnet mask.
  • geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□
    So harsh!
    I am not being negative or defensive. I also don't need to have my message be deemed crap.

    I also read, your comment about not minding questions, so why bring up that I asked a similar question twice at all?

    Simply, I did not understand a part of the TechNotes and other resources, and I only wanted clarification on what I was reading and how to apply it to the CompTia Objectives.

    Besides, aren't we all hear to help each other?
    Ciao and Meow


  • eurotrasheurotrash Member Posts: 817
    a bed has two sides...
    witty comment
  • geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□
    Ciao and Meow


  • WebmasterWebmaster Admin Posts: 10,292 Admin
    I also read, your comment about not minding questions, so why bring up that I asked a similar question twice at all?
    Maybe because I'm trying to help. icon_rolleyes.gif As I clearly stated, I don't mind an additional questions in the forums, that's what they are for. However, that doesn't mean I won't be doing my job and point you to posts or TechNotes that already cover the answers to your questions, to make sure you haven't missed those for any reason, to make sure you understand the topic, and hence help you pass.
    Besides, aren't we all hear to help each other?
    What do you think I'm doing here and have been doing for 3 years every day...? And while you're at it, why do you think I created these forums in the first place.
    Besides, haven't you heard the saying, "The only stupid question is the one that isn't asked."? Also, I thought that is what forums were for... questions.
    That's defensive and negative while I was not attacking, just trying to help.
  • geekgodesgeekgodes Member Posts: 16 ■□□□□□□□□□
    To be honest...
    that's all I ever wanted help and some additional clarification.

    Thanks for your patience and help.
    Ciao and Meow


  • WebmasterWebmaster Admin Posts: 10,292 Admin
    And that's exactly what you can expect, and includes suggestions to read certain sections of my TechNotes, other sources, previous topics, and advice to use the search option to see if a question has been posted before etc.etc. Which all in no way means I 'mind' the post/question itself. You would have received a PM with the subject 'removed post' if that were the case. And I should have picked a better word than 'crap', I guess it looks a lot more harsh when you read it than when I typed it. Don't hesitate to ask any other questions.
  • porengoporengo Member Posts: 343
    Johan, has anyone ever told you that you don't "crap" where you "forum?" icon_lol.gif
  • jsobrinogjsobrinog Member Posts: 54 ■■□□□□□□□□
    Maybe the topic for this replay can answer the question.

    Subnetting is an important part of a good network logical topology.
    What do this means? , there are some net admins that use a complet 192.168.0 subnet for their networks, but through time, they figure that their nodes are reducing and they experience heavy traffic. Here is where subnetting comes, oferring a discret ip designation and creating diferente "lans" "segments" "subnetS" that interact only in their own borders.

    :S hope this can explain.

    P.D didnt read the answers for this post , i just reply it jejejeje sorry.
  • davidhdavidh Member Posts: 11 ■□□□□□□□□□
    I didnt get this either when i first started learning about subnetting. You have to look at it from a bigger perspective, most of us are used to configuring small, single site lans with class c networks and/or not understanding how routers work.

    **You will need to have an understanding of how subnetting works and how to do it. Understanding the binary conversion of an ip addresses is crucial to comprehending this**

    The purpose of subnetting is to minimize wasting network addresses when splitting networks.

    You have been assigned a Class C range of IP addresses

    You have 180 servers. These computers generate heavy traffic and you've decided to seperate the computers into 6 seperate network segments.

    A router on a remote network receives a packet, and sees that it is destined for ip and starts sending it across the network.

    Once it reaches the router that controls network 192.168.0.x (the main incomming router for your entire class c network), the router looks at its routing table...

    Network ID IP addresses within this network ip's x.x.x.33 - x.x.x.62 ip's x.x.x.65 - x.x.x.94 ip's x.x.x.97 - x.x.x.126 ip's x.x.x.129 - x.x.x.158 ip's x.x.x.161 - x.x.x.190 ip's x.x.x.193 - x.x.x.222

    because ip address in binary is
    11000000 10101000 00000000 01000001
    and the router for 192.168.0.x knows that this ip address is on a network with a subnet of (configured on the router)
    11111111 11111111 11111111 11100000

    it compares the ip and the subnet mask
    11000000 10101000 00000000 01000001
    11111111 11111111 11111111 11100000
    the router can see that the network id ends after the first 3 bits in the ip address (see below)
    11000000 10101000 00000000 010 00001
    and according to the first 3 bits in the last octet of the ip address, (010) which is 64 in binary, the router knows to send the packet to the router that controls network (look at the above routing table)

  • igor_yuriigor_yuri Member Posts: 12 ■□□□□□□□□□
    Hi~ you were asking where did X from "2^x-2" came from? It's actually a Base 2 binary. Lets say a Class C

    N.N.N = Network ID - 192.168.67
    H = Host = .200 = 8 bits

    Now, remember each octet represents 8 bits. Since, its a Base 2 getting the number of bits from host part which is 8. Therefore, 2^8 - 2 = 254 Host ; now you might wanna know where did we get the -2 part. It's a default value for 1 and 0. Zero(0) for Network ID and One(1) for Broadcast ID.

    Cass B

    N.N = Netowrkd ID - 130.70
    H.H - Host ID - 66.195 = 16 bits

    Simple as 2^16 - 2 = 65,534 Host

    Now, solve for Class A. You should have 16,777,214 Host

    I hope this helps. Wish me luck tomorrow for my Network+ exam!
  • micro420micro420 Member Posts: 3 ■□□□□□□□□□
    I haven't read through this whole post with replies, but to answer your question in respect to the Network+ exam, which I just took today and had this very same question, the answer was something to the effect of "consolidation of small networks from one big network."

    The other responses, which were not correct were:
    increase bandwidth
    decrease congestion (maybe...)
    ????? this option was something ridiculous and off the wall

    Hope this helps
  • TechJunkyTechJunky Member Posts: 881
    Try this link...

    Should help explain a little better.

    I made this when I was having a hard time with subnetting.
  • elathropelathrop Member Posts: 88 ■■□□□□□□□□
    That was invigorating! :)

    Here is the way I would answer the question “What is the purpose of subnetting?”

    To break up your network into smaller segments and to be better able to organize collision domains.

    IE, You could keep your subnets on separate switch ports and more easily manage or recognize them by their IP addresses. It certainly is only a matter of organizing things. A switch alone would seperate collision domains but you may want an easy way of knowing which one is which. You could seperate by departments or classes, etc.

    I’m sure nothing like that will be on the test, but I had fun taking a crack at it. :D

    Ciao and Bow Wow
    Webmaster for and
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