Subnetting - usable addresses

urstuffplz1urstuffplz1 Member Posts: 76 ■■■□□□□□□□
in Off-Topic
Hi all,

I have an assignment for my university course and one of the questions has asked:

150.16.0.0 / 18

How many subnets and usable subnets are there?

I am lead to believe that there is 4 subnets (10, 11, 01, 00 binary) and all are usable. My friend believes that only 2 are usable and has found sources online stating this, with 2 being unusable.

Can someone just confirm please, and if I am wrong, just explain why?

Kind regards,

Stephen
2018 Goals: CCNP Route 300-101[X], CCNP Switch 300-115[X], CCNP T'Shoot 300-135[X], VCP-DCV 6[], 70-412[], 70-413[], 70-414[]
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Comments

  • linuxabuserlinuxabuser Member Posts: 97 ■■□□□□□□□□
    You have to be more specific. You determine the size of the subnet you want by how many hosts need to fit in it.

    That /18 could be one big usable subnet. It could also be a bunch of /30s.

    There has to be more information to answer the question.
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  • kohr-ahkohr-ah Member Posts: 1,277
    It really depends.
    Is your professor counting that you get to break it down further via CIDR?
    Because in that case I can take you /18 and divide it up into /30s and get WAY over 4 subnets.

    If you are talking basics (Like lets not break it down) then yes 4 are usable. It is called IP Subnet Zero.

    150.16.0.1 - 150.16.63.254
    150.16.64.1 - 150.16.127.254
    150.16.128.1 - 150.16.191.254
    150.16.192.1 - 150.16.255.254

    Each one gets 16,382 usable hosts.(There is a total of 16,834 addresses but remember to use the 2^n-2 rule = 16382)
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  • OctalDumpOctalDump Member Posts: 1,722
    I still don't understand the question. If it is related to Subnet Zero, I'd refer you to RFC 1878 which obsoleted subnet zero more than 20 years ago.

    So, for 150.16.0.0/18 it could be divided into many (4096) /30 subnets with 2 usable addresses each, or into two /19 networks with 8190 usable addresses.

    Or is the question asking for network 150.16.0.0, how many /18 subnets can you create? Since 150.16.0.0 is a class B, it's /16, which allows 4 /18 sub-networks, as kohr-ah describes.
    2017 Goals - Something Cisco, Something Linux, Agile PM
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  • urstuffplz1urstuffplz1 Member Posts: 76 ■■■□□□□□□□
    Hi all,

    Thank you for all the replies.

    It was just asking how many subnets for that range, not subnetting further. The CIDR has to remain /18 for the 150.16.0.0 address.

    I am write in my thinking that for this CIDR and address you can have 4 subnets, each with 16,382 usable hosts in each.

    Stephen
    2018 Goals: CCNP Route 300-101[X], CCNP Switch 300-115[X], CCNP T'Shoot 300-135[X], VCP-DCV 6[], 70-412[], 70-413[], 70-414[]
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  • d4nz1gd4nz1g Member Posts: 464
    If he considers that 150.16.0.0 is a class B address, where 150.16 is the network portion and 0.0 (low order 16 bits) are the host portion. Therefore, the mask for this class is /16.

    In the range 150.16.0.0/18, you will keep the 150.16 network portion with 16 bits, and 2 bits for the subnet portion. Remaining only 14 for the host portion.

    So, as OctalDump mentioned: Since 150.16.0.0 is a class B, it's /16, which allows 4 /18 sub-networks
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  • vasyvasyvasyvasy Member Posts: 68 ■■■□□□□□□□
    urstuffplz1 wrote: »
    The CIDR has to remain /18 for the 150.16.0.0 address.
    Stephen
    There is no such thing as subnetting, while leaving the SM the same :)
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