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TOPIC: Why A Class C address with a /25 mask only borrows one bit

Why A Class C address with a /25 mask only borrows one bit 10 years 6 months ago #13745

  • ccnx
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Hi all

can some1 tell me that why A Class C address with a /25 mask only borrows one bit ??

As i knew,
For Class C: /25 => cccccccc.cccccccc.cccccccc.C000000 = 1 bit
For Class A: /25 =>aaaaaaaa.AAAAAAAA.AAAAAAAA.A0000000 = 17

So there should be 17 bits rather than 1 bit borrowed from the rest of unassigned bits
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subnetting... 10 years 6 months ago #13746

  • Arani
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hi there mate,
well, to start with look at the default subnet mask for a class C IP address.

it is 255.255.255.0, which means, that the first 24 bits of the IP address remains the same.

now if you define a subnet mask as /25, it means that an extra 1 bit out of the total 32 bits will remain unchanged while the addressing scheme is declared. this in turn means that you are dividing the entire network into 2 division. now look at the IP addresses carefully: (imagine the zeros to be of some value)

00000000 . 00000000 . 00000000 . 00000000

the first 24 bits of a class C address is the network ID, only the last 8 bits form the host ID. now for subnetting, even this 8 bits will now be divided into a 1 bit subnet ID and a 7 bit host ID. remember i said, by defining a /25 subnet mask, you are dividing the network into 2 portions, this is how:(again, imagine the zeros to be of some value)

the first network will be:
00000000 . 00000000 . 00000000 . (0) 0000000

the second network will be:
00000000 . 00000000 . 00000000 . (1) 0000000

so you can very well see, it is by the rule of subnetting that a subnet mask shown as /25 will take an extra bit from the host address section, to define the subnet ID, and the rest 7 bits will be used to define the host ID.

for a class A IP address, the default is 255.0.0.0, so you can imagine by using a /25 subnet mask you are dividing the network into 2 raised to the power of 17 number of sub - networks which calculates to a mind boggling 131072 networks!!!! you also have to keep in mind that the network address of a Class A IP address consists of the first 8 bits.

this is not the case for a Class C IP address. the network ID is already the first 24 bits. so a subnet mask of /25 will allow the masking for only 1 bit. hence you can only utilize one bit, and not 17.

Hope this solves your issue, and I am sure others will agree with me regarding this explanation.
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Re: Why A Class C address with a /25 mask only borrows one bit 10 years 6 months ago #13750

  • ccnx
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:roll:

Thanks you so much Arani, you just had a very good explanation for me!!!
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less trouble 10 years 5 months ago #13756

  • Arani
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actually you can read through the subnetting notes here on firewall.cx and you will learn a great deal. i learnt most about subnetting through these notes. you can find them under NETWORKING ->PROTOCOLS->INTERNET PROTOCOL SUBNETTING...

happy reading!!! :lol:
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