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  1. Senior Member
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    #1

    Default Hi, How many subnets and hosts per subnet can you get from the network 172.20.0.0/28?

    I tried using the 2^n and (2^n)-2 formula but I'm not getting
    4096 subnet and 14 hosts answer.

    Please help.
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  3. Senior Member luke_bibby's Avatar
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    #2
    172.20.0.0/28 or rather 172.20.0.0/255.255.255.240

    Class B address.
    Class B default mask = 255.255.0.0

    172.20.0.0 is subnetted by borrowing 12 bits from the host portion of the address (28 - 16 = 12).

    Number of subnets = 2^12 = 4096
    Number of usable hosts = (2^4) - 2 = 16 - 2 = 14
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  4. Junior Member closetgeek's Avatar
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    #3

    Default Heres your answer....

    16 subnets and 14 host -I'm pretty good at this so.... ask away....lol
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  5. Junior Member closetgeek's Avatar
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    #4
    Quote Originally Posted by SurferdudeHB View Post
    I tried using the 2^n and (2^n)-2 formula but I'm not getting
    4096 subnet and 14 hosts answer.

    Please help.
    The secret is in your subnet mask....once you find your mask the rest is easy... I did it in my head.Google subnetting secrets this is a very good way to learn subnetting.
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  6. Senior Member
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    #5
    Quote Originally Posted by luke_bibby View Post
    172.20.0.0/28 or rather 172.20.0.0/255.255.255.240

    Class B address.
    Class B default mask = 255.255.0.0

    172.20.0.0 is subnetted by borrowing 12 bits from the host portion of the address (28 - 16 = 12).

    Number of subnets = 2^12 = 4096
    Number of usable hosts = (2^4) - 2 = 16 - 2 = 14
    Got it thanks!
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  7. Constantly learning
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    #6
    Quote Originally Posted by closetgeek View Post
    16 subnets and 14 host -I'm pretty good at this so.... ask away....lol
    It is not classless and neither a class C address
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  8. Senior Member blackninja's Avatar
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    #7
    Quote Originally Posted by closetgeek View Post
    16 subnets and 14 host -I'm pretty good at this so.... ask away....lol
    Must try harder
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  9. Senior Member sina2011's Avatar
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    #8
    Firstly I know this an old thread but Ive been looking for a good explanation on this part of subnetting and you guys have made me 1 step closer to understanding subnetting completley.
    I Just have one Question for:

    @Luke Bibby

    where did you get (Number of usable hosts = (2^4) from in the second part of your explanation I cant seem to figure it out.


    Cheers.
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  10. Senior Member KrisA's Avatar
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    #9
    Quote Originally Posted by sina2011 View Post
    Firstly I know this an old thread but Ive been looking for a good explanation on this part of subnetting and you guys have made me 1 step closer to understanding subnetting completley.
    I Just have one Question for:

    @Luke Bibby

    where did you get (Number of usable hosts = (2^4) from in the second part of your explanation I cant seem to figure it out.


    Cheers.
    I am not Luke - However... that is the number of "Off bits" aka the 0's ...

    /28 = 11111111.11111111.11111111.11110000 <- 4 Off bits..... Just count ( you will starting thinking of it binary after awhile) the 0's
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  11. Senior Member
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    #10
    ^ What he said. But rather than doing it in binary...if you have 32 bits, and 28 are used for the network address, than means 32 - 28 = 4 bits for hosts.
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  12. Censorship is Un-American JockVSJock's Avatar
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    #11
    I'm confused on how the answer for this was gotten too. I'm using Chris Bryant's method of subnetting and not getting the same answer:

    IP Address: 172.20.0.0
    Default Mask: 255.255.0.0
    Subnet Mask: 255.255.255.224

    I convert the default mask and the subnet mask to binary:

    Default Mask = 11111111.11111111.00000000.00000000
    Subnet Mask = 11111111.11111111.11111111.11100000

    Using his method I take the bits turned on under the subnet mask (2^11) to find the subnets which gives me = 2048
    And again, using his method to find the host, I use the following (2^11-2), which gives me = 2046

    Not saying I'm correct, I'm just using the method that works for me the best and trying to generate discussion and understand what is going on.

    thanks
    Last edited by JockVSJock; 09-19-2011 at 09:38 PM. Reason: Wanted to bold NOT
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  13. Questionably Benevolent Moderator Slowhand's Avatar
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    #12
    Quote Originally Posted by JockVSJock View Post
    I'm confused on how the answer for this was gotten too. I'm using Chris Bryant's method of subnetting and not getting the same answer:

    IP Address: 172.20.0.0
    Default Mask: 255.255.0.0
    Subnet Mask: 255.255.255.224

    I convert the default mask and the subnet mask to binary:

    Default Mask = 11111111.11111111.00000000.00000000
    Subnet Mask = 11111111.11111111.11111111.11100000

    Using his method I take the bits turned on under the subnet mask (2^11) to find the subnets which gives me = 2048
    And again, using his method to find the host, I use the following (2^11-2), which gives me = 2046
    A /28 mask would give 12 network bits, not 11:
    Subnet Mask = 11111111.11111111.11111111.11110000

    That means the subnet mask is 255.255.255.240.

    With 12 bits, you'd have 2^12 which equals 4096. For the hosts, you'd have (2^4)-2, which makes 14 hosts on each subnet.

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  14. Censorship is Un-American JockVSJock's Avatar
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    #13
    Quote Originally Posted by Slowhand View Post
    A /28 mask would give 12 network bits, not 11:
    Subnet Mask = 11111111.11111111.11111111.11110000

    That means the subnet mask is 255.255.255.240.

    With 12 bits, you'd have 2^12 which equals 4096. For the hosts, you'd have (2^4)-2, which makes 14 hosts on each subnet.
    I don't get it.

    The original subnet mask is 255.255.255.224, isn't that /27?

    I don't understand where you are getting /28.
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  15. Questionably Benevolent Moderator Slowhand's Avatar
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    #14
    Quote Originally Posted by JockVSJock View Post
    I don't get it.

    The original subnet mask is 255.255.255.224, isn't that /27?

    I don't understand where you are getting /28.
    The OP asked for /28 in the title of the thread: Hi, How many subnets and hosts per subnet can you get from the network 172.20.0.0/28?

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  16. Member
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    #15
    To find the number of subnets use 2^n for the on bits
    172.20.0.0
    255.255.255.224
    11111111.11111111.11111111.11100000 (11 bits on) so 2^11=2048 subnets

    To find the number of host per subnet use 2^n-2 for the off bits
    172.20.0.0
    255.255.255.224
    11111111.11111111.11111111.11100000 (5 bits off) so 2^5-2=30 host per subnet
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  17. Censorship is Un-American JockVSJock's Avatar
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    #16
    Quote Originally Posted by Slowhand View Post
    The OP asked for /28 in the title of the thread: Hi, How many subnets and hosts per subnet can you get from the network 172.20.0.0/28?
    Ya, I really f****d that one up.

    Sorry.
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  18. Questionably Benevolent Moderator Slowhand's Avatar
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    #17
    Heh, no worries. At least it was here on TE and not at work you got the wrong number. . . unlike me, who worked an entire graveyard shift, called Cisco SmartNet support twice, and probably tore out half my hair in frustration before realizing that it wasn't my VPN tunnel or my firewall settings that were wrong, it was the address and mask on a subinterface.

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  19. Senior Member
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    #18
    Quote Originally Posted by Slowhand View Post
    Heh, no worries. At least it was here on TE and not at work you got the wrong number. . . unlike me, who worked an entire graveyard shift, called Cisco SmartNet support twice, and probably tore out half my hair in frustration before realizing that it wasn't my VPN tunnel or my firewall settings that were wrong, it was the address and mask on a subinterface.
    Eh, at least you weren't like closetgeek (3rd post) who claims he's really good at subnetting and then proceed to give the totally wrong answer
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    #19
    i have a question. Answer me please. Question:How many subnet and hosts per subnet can you get from the network 174.20.0.0/23 Remember for host part at least 2 bits are required?
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  21. Senior Member
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    #20
    Quote Originally Posted by Steadfast Girl View Post
    i have a question. Answer me please. Question:How many subnet and hosts per subnet can you get from the network 174.20.0.0/23 Remember for host part at least 2 bits are required?
    Download this to check. IP Subnet Calculator

    Subnet mask in binary:
    11111111.11111111.11111110.00000000 (255.255.254.0)

    Its /23 so there are 23 network bits.

    This leaves us with 9 host bits (0s).. So 2^9=512 and minus the 2 so 510.
    This leaves us with 7 host bits (1s) in the octet we're working with ..So 2^7=128.

    128 subnets
    510 hosts.
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    #21
    Quote Originally Posted by Steadfast Girl View Post
    i have a question. Answer me please. Question:How many subnet and hosts per subnet can you get from the network 174.20.0.0/23 Remember for host part at least 2 bits are required?
    The answer is:

    (2^9)-2=510
    510 hosts per subnet

    (2^7)=128
    128 subnet
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  23. Junior Member Registered Member
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    #22
    thanks for answering my question. GOD bless u allz.
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  24. Senior Member
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    #23
    Quote Originally Posted by Steadfast Girl View Post
    thanks for answering my question. GOD bless u allz.
    most welcome
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    #24
    Quote Originally Posted by luke_bibby View Post
    172.20.0.0/28 or rather 172.20.0.0/255.255.255.240

    Class B address.
    Class B default mask = 255.255.0.0

    172.20.0.0 is subnetted by borrowing 12 bits from the host portion of the address (28 - 16 = 12).

    Number of subnets = 2^12 = 4096
    Number of usable hosts = (2^4) - 2 = 16 - 2 = 14
    This is correct
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  26. Junior Member
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    #25
    Quote Originally Posted by rphann View Post
    The answer is:

    (2^9)-2=510
    510 hosts per subnet

    (2^7)=128
    128 subnet
    I couldn't have said it better.
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