Subnet confusion

I get most of the subnetting things, I think.
For example how to calculate how many subnets / hosts / Network ID / broadcast ...
Having said that what confuses me is if the subnet is a full octet!

I don't understand how to calculated subnets based on subnets like

10.1.0.0
255.255.255.0

I'm NOT confused (I don't think) about class or unclassfull / cidr stuff. This address is a common address used by many routers and companies. It’s a class B but based on the mask it’s a class C. But that doesn’t matter cause its cidr, and classes are not used any more, (basically).

I'm just use to doing supernetting (I think its call) but confused when you
have an IP that is a full Octet.

for example
Here is my math on the above address.
10.1.0.1
0000001010.00000001.00000000.00000001 >IP 10.1.0.1
1111111111.11111111.11111111.00000000 >Subnet Mask 255.255.255.0
0000001010.00000001.00000000.00000000 Network IP 10.1.0.0 using Anding.
.11111111 I just add 1's to host bits to get broadcast
address. 10.1.0.255

I think that there are 8 subnet bits in the third octet and 8 host bits in
the 4th octet, that should give me 256 subnets (minus 2) and 256 hosts
(minus 2)

HOWEVER when I look at an ip subnet calculator I see that there are 65536
subnets available and 256 hosts. I'm confused where the 65536 subnets comes
from (other than its 2^16=65536) Where did the 16 bits come from for the
subnet.

Thanks.
4 answers Last reply
More about subnet confusion
  1. stevwolf said:
    I think that there are 8 subnet bits in the third octet and 8 host bits in
    the 4th octet, that should give me 256 subnets (minus 2) and 256 hosts
    (minus 2)


    there are 8 subnet bits in the third octet and 8 host bits in
    the 4th octet, that should give you 256 subnets and 256 hosts
    (minus 2)


    stevwolf said:
    HOWEVER when I look at an ip subnet calculator I see that there are 65536
    subnets available and 256 hosts. I'm confused where the 65536 subnets comes
    from (other than its 2^16=65536) Where did the 16 bits come from for the
    subnet.


    It should be 2^16 - 2 = 65534


    http://en.wikipedia.org/wiki/IP_address
  2. Use a CIDR calculator. A subnet calculator is looking at the class of address and you mask to get your results.

    http://www.subnet-calculator.com/cidr.php
  3. Perhaps thats it. I should use a cidr calculator. Some caclulators are a little fuzzy on this aspect.
    thanks.
  4. stevwolf said:
    I get most of the subnetting things, I think.
    For example how to calculate how many subnets / hosts / Network ID / broadcast ...
    Having said that what confuses me is if the subnet is a full octet!

    I don't understand how to calculated subnets based on subnets like

    10.1.0.0
    255.255.255.0

    I'm NOT confused (I don't think) about class or unclassfull / cidr stuff. This address is a common address used by many routers and companies. It’s a class B but based on the mask it’s a class C. But that doesn’t matter cause its cidr, and classes are not used any more, (basically).

    I'm just use to doing supernetting (I think its call) but confused when you
    have an IP that is a full Octet.

    for example
    Here is my math on the above address.
    10.1.0.1
    0000001010.00000001.00000000.00000001 >IP 10.1.0.1
    1111111111.11111111.11111111.00000000 >Subnet Mask 255.255.255.0
    0000001010.00000001.00000000.00000000 Network IP 10.1.0.0 using Anding.
    .11111111 I just add 1's to host bits to get broadcast
    address. 10.1.0.255

    I think that there are 8 subnet bits in the third octet and 8 host bits in
    the 4th octet, that should give me 256 subnets (minus 2) and 256 hosts
    (minus 2)

    HOWEVER when I look at an ip subnet calculator I see that there are 65536
    subnets available and 256 hosts. I'm confused where the 65536 subnets comes
    from (other than its 2^16=65536) Where did the 16 bits come from for the
    subnet.

    Thanks.



    Go and ask that Guy Wally to answer those q's from teh comic strip or Dogbert! 255 is mass broadcast to all 254 nodes in the sub net. Goto wikipedia and look up subnets A-D and you'll figure it out or buy a Network+ book for 20bucks!
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