Easy Guide to Subnetting

Discussion in 'Networking and Internet' started by XavierYZ, Jan 30, 2003.

  1. XavierYZ

    XavierYZ

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    Easy lesson to Subnetting By XavierYZ

    Sub netting is required when you have a large network of computers!

    If we have a large network of computers and chose to use the default subnet of a private range what happens is the network broadcasts its entire request across the network. This creates a decrease performance.

    So what subnet masks allow us to do is break a network into segments, but still allow the entire network to function as one.


    Breaking down subnet into binary

    Class A 255.0.0.0 = 11111111.00000000.00000000.00000000
    Class B 255.255.0.0 =11111111.11111111.0000000.00000000
    Class C 255.255.255.0 =11111111.11111111.11111111.00000000


    /24 are the CIDR -- Classless InterDomain Routing. CIDR was invented several years ago to keep the Internet from running out of IP addresses.

    E.g. 192.168.1.1/24 is the default for IP/Subnet for a class C network

    /24 represents Bits or 1’s…we should all know that a Class C subnet is 255.255.255.0, but how do you figure this out.

    We know that there are 8bits to each of the 4 octets of an IP address

    11111111.11111111.11111111. < 24 1’s represent /24 of the network side
    Remaining octet will have all zeros which is the host side of a subnet > .00000000

    *Note Subnets Start with 1’s and end in 0’s, it will never look like this 11110111.01010000.00000000.00000000











    *This chart is hard to read~on the forum, I will post the html when I upload the file at home

    192 224 240 248 252 254 255
    128 64 32 16 8 4 2

    Small numbers represent sums of each bit of an octet e.g., 11111000 = 128+64+32+16+8+0+0+0 or because there are 5 1’s we can just look at the chart and know that 5 1’s are 248

    EG. 192.168.1.1/17 = /18 1’s at the front of our subnet
    Binary format: 11111111.111111111.11000000.00000000
    Decimal format: 255.255.192.0

    Now what does this subnet mean in reality?

    Going back to the binary format

    11111111.111111111.11000000.00000000

    1’s represent the number of networks you can have
    0’s represent the number of hosts per network

    to determine the number of hosts we use this formal

    2 to the power of x - 2 = hosts X = #of 0’s

    2 to the power of 14 - 2 = 16382
    *We subtract 2 because we don’t use 0 or 255 in the last subnet of an IP address

    So with an IP of 192.168.1.0/18 the subnet is 255.255.192.0
    This network can have 16382 computers in one segment of a network.














    EG scenario.

    You are the network administrator of a large network using TCP/IP as its only transport protocol. You need to design a network that will handle 25 subnets with a maximum of 2000 hosts. What subnet will you use?

    A: 255.255.255.0
    B: 255.255.240.0
    C: 255.255.248.0
    D: 255.255.240.0

    To determine the answer use our host formula

    2 to the power of x-2 = hosts X = #of 0’s

    We know that hosts = 2000

    We need to determine how many 0’s in our subnet we need if x = 0’s
    Find out how many powers of 2 it takes to reach a maximum of 2000 hosts!

    2 to the power of 11 - 2 =2046

    Ok! Now we need to put this into a subnet mask. We now know that 11 0’s are need in our subnet mask 0’s represent Hosts

    32bits (0’s and 1’s) – 11 bits (0’s) = 21 1’s

    11111111.11111111.11111000.00000000 this is our binary format, Transfer it to decimal form and you have 255.255.248.0

    So to answer this question

    A subnet mask of 255.255.248.0 or 192.168.1.0/21 will allow us to have at least 2046 hosts.


    If anyone finds this txt format difficult to understand I can provide a doc for you
     

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    XavierYZ, Jan 30, 2003
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  2. XavierYZ

    James Photojournalist VIP Member

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    wow, this is great.. very helpful.

    I wish I had more computers, I only have 4, 3 can't be used for servers, or firewalls- they are peoples PC's :cry:
     
    James, Jan 30, 2003
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  3. XavierYZ

    XavierYZ

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    that info will be very usefull for the NEt infrastructure cert
     
    XavierYZ, Jan 30, 2003
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  4. XavierYZ

    James Photojournalist VIP Member

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    This is going to come in handy, I'm taking a 'mentor' class at an ISP that I work for, on IP and subnetting.
     
    James, Feb 21, 2003
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  5. XavierYZ

    arziman

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    Sorry, I think you are wrong. 192.168.1.1 is a class C address, so the smaller subnet mask possible is /24. It could be bigger (/24, /25, ... , /32) but cannot be smaller than the defaul subnet mask.
     
    arziman, Mar 22, 2013
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