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TOPIC: Subnet ID using host bits, i`m lost.

Subnet ID using host bits, i`m lost. 10 years 7 months ago #13136

  • Jiggy
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Hi all great site.

www.firewall.cx/ip-subnetting-mask-effect.php

I`m reading the above link on Class C Classless IP Address and I`m finding it hard to understand how the subnet ID is made, from reading the page the pic shows me the subnet ID is made up from the first 3 bits of the IP address and the first 3 bits of the subnet mask.

128 64 32
0 0 0 - first 3 bits of IP
1 1 1 - first 3 bits of mask

My questions.

If you take bits from the IP wont it change it ?
What do you have to do to make a subnet ID from the 6 bits?
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Re: Subnet ID using host bits, i`m lost. 10 years 7 months ago #13139

  • havohej
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Hi.

The question I uderstand is what happens if you borrow 6 bits from a class c address???

please tell Me if I it was not the question.

I will answer anyway.

If you borrown 6 bits from the Host portion of a class C address it belong to a /30 mask, where you have 4 hosts, but only 2 are usable, for the rule that the first and the last are subnet Id and broadcast.

so , converting to binary it looks like this:


128 64 32 16 8 4 | 2 1
128 192 224 240 248 252 | 254 255

It belongs to a /30 mask, or 255.255.255.252 used only for wan point to point addressing.
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Re: Subnet ID using host bits, i`m lost. 10 years 7 months ago #13228

  • Jiggy
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Hi havohej,

Thank you for your reply, after reading the next page on from the one I posted it cleared it up, i`m now starting to get a better understanding of the topic.
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