Current Events > Anyone here know anything about CCNA or, like, subnetting in general?

(+) Yeah! (+)

inb4 do your own homework

this isn't worth any marks, it's review for a test that's like 2 weeks away that I took a peak at.

This is just, like, networking 101. I'll give one question I'm having trouble with for now:

Given the subnet mask of 255.255.254.0 and a host address of 152.202.64.51, find the subnetwork number to which this host belongs.

Here's how I think it would go:

That subnet mask looks like this in binary

11111111.11111111.111111110.00000000

That means there's only one bit in the second to last octet available for subnet addresses. In other words, two subnets with 255 addresses each. This jives with everything I read online, too. Since his host address is 64.xxx, he's on subnet 0.

HOWEVER, the review says this is the answer:

host belongs to subnet 32 (7 bits borrowed)

Like, how does he get this? 7 bits borrows? 255.255.254.0 is 9 bits borrows, everything I've read online say that it's 9 borrows bits and... it just makes no sense. :\

The next question is basically the same thing:

Given a host address of 132.20.0.193 and that you borrowed 11 bits, find the subnet mask and the wire address of the subnet to which this host belongs.

Borrows 11 bits means /21, or 255.255.248.0. That means he has a subnet of size 8. Since his middle octet is 0, he is again on the zeroth subnet and his wire address is 132.20.0.0.

The answer on the review:

SM = 255.255.255.224
wire address = 132.20.0.192

Like... no? Not at all? Like, absolutely no fucking way not at all?

I tried e-mailing the prof but he swears by them being correct... I just don't understand, man. :\
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For problem 1:
1) You've got the binary representation correct
2) 152.202.64.51 would be a class B public address (128.0.0.0 - 191.255.255.255)
3) Class B public addresses have a default subnet mask of 255.255.0.0 (11111111.11111111.00000000.00000000)
- So, when it mentions "7 bits borrowed," I believe's borrowing 7 network bits for 255.255.254.0 in relation to the IP 152.202.64.51
4) "Host belongs to subnet 32"
- You're correct in that the number in the 3rd octet is 64.

"Host belongs to subnet 32"

You would essentially find this by taking the lowest possible subnet address and then listing them. Since we know the default subnet mask for a class B address is 255.255.0.0, then you'd essentially start at 0 in the 3rd octet and then count the subnets up until you get to the range the IP address lives in.

With a 255.255.254.0 subnet mask (/23), you have an increment of 2 in the 3rd octet. The lowest 1 bit in a subnet mask is the increment.

Subnet 0: 152.202.0.0
Subnet 1: 152.202.2.0
Subnet 2: 152.202.4.0
Subnet 3: 152.202.6.0
Subnet 4: 152.202.8.0
Subnet 5: 152.202.10.0

And so on, and eventually you'd get to...

Subnet 32: 152.202.64.0

I'm assuming the teacher is teaching subnetting in a different way than I learned it, but it all works out the same in the end.
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For Problem 2:

Again, I'm assuming he means borrowing network bits (1), not host bits (0). So with an class B IP address of 132.20.0.193, the default subnet mask is 255.255.0.0.

255.255.0.0 = 11111111.11111111.00000000.00000000
Borrowing 11 network bits:

11111111.11111111.11111111.11100000 = 255.255.255.224.
The lowest 1 bit here is in the 4th octet, and that 3rd bit position in the 4th octet represents 32, so that's your increment.

Subnet 0: 132.20.0.0
Subnet 1: 132.20.0.32
Subnet 2: 132.20.0.64
Subnet 3: 132.20.0.96
Subnet 4: 132.20.0.128
Subnet 5: 132.20.0.160
Subnet 6: 132.20.0.192

And I'm also assuming by "wire address," he means "subnet address." The 1st IP address in every subnet is called the subnet address, and the last IP address is the broadcast address. 132.20.0.193 belongs to subnet #6, and the subnet address (wire address) is 132.20.0.192.

I've never heard it referred to as a wire address, but that's apparently just a different way to refer to it... I think.
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Subnetting is one of those things that feels like youre randomly moving around nonsense spaghetti noodles when you first learn it and then becomes like the easiest thing in the world after it finally clicks
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ultimate reaver posted...

Subnetting is one of those things that feels like youre randomly moving around nonsense spaghetti noodles when you first learn it and then becomes like the easiest thing in the world after it finally clicks

Exactly that. When I 1st started studying for my CCENT, I thought subnetting would be the hardest part of everything, but... It's just easy stuff now.
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Subnetting is annoying more than anything imo
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So I hopped on to the CCNA discord and learnt myself a bit.

Apparently this is classful addressing that my teacher is teaching us. Which is out of style and nigh-on useless. But it makes sense in that context.

What I'm trying to do is CIDR addressing. So I guess I should go, uh, learn classful addressing so I don't fail the test. :\
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