Subneting ?

themagicone

I thought I got the whole thing down till now... (it could be labsim, I've found many errors with their CCNA training)

If I have say a /22 or /23 network, what is the proper step for the subnets?
I get 2 for /23 and 4 for /22. That would give me .0.0, .2.0, .4.0 and 0.0, 4.0, 8.0, respectively. Or is that wrong? Labsim is telling me that it is 4 and 8.

I write myself a chart to help
Bit 1 1 1 1 1 1 1 1
Count Number 128 64 32 16 8 4 2 0
Mask Number 128 192 224 240 248 252 254 255
Host/Net 126 65 30 14 6 2 1 0
Subnets 2 4 8 16 32 64 128 192

Sorry, kinda of hard to put it in here.

So going by that chart a /23 would be in the 2nd to last column or 2 count.

Am I way off?

Jollycork

here's a link:

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html


it's a really good writeup for subnetting.

here's an except from that subnetting writeup:

Just memorize how many items you get for each number of bits (subtract two for hosts)

000000000001 = 2 (1)
000000000011 = 4 (2)
000000000111 = 8 (3)
000000001111 = 16 (4)
000000011111 = 32 (5)
000000111111 = 64 (6)
000001111111 = 128 (7)
000011111111 = 256 (8 )
000111111111 = 512 (9)
001111111111 = 1024 (10)
011111111111 = 2048 (11)
111111111111 = 4096 (12)

themagicone

Ah thanks. I think labsim has yet another error. That chart is off on the subnet step number. I'd be 4 and 8 according to that other post.The chart the gave was 2 and 4, offset to left by one.

instant000

themagicone wrote: »

I thought I got the whole thing down till now... (it could be labsim, I've found many errors with their CCNA training)

If I have say a /22 or /23 network, what is the proper step for the subnets?
I get 2 for /23 and 4 for /22. That would give me .0.0, .2.0, .4.0 and 0.0, 4.0, 8.0, respectively. Or is that wrong? Labsim is telling me that it is 4 and 8.

I write myself a chart to help
Bit 1 1 1 1 1 1 1 1
Count Number 128 64 32 16 8 4 2 0
Mask Number 128 192 224 240 248 252 254 255
Host/Net 126 65 30 14 6 2 1 0
Subnets 2 4 8 16 32 64 128 192

Sorry, kinda of hard to put it in here.

So going by that chart a /23 would be in the 2nd to last column or 2 count.

Am I way off?

/30 is 255.255.252.0
/29 is 255.255.248.0
/28 is 255.255.240.0
/27 is 255.255.224.0
/26 is 255.255.192.0
/25 is 255.255.128.0
/24 is 255.255.255.0
/23 is 255.255.254.0
/22 is 255.255.252.0
/21 is 255.255.248.0
/20 is 255.255.240.0
/19 is 255.255.224.0
/18 is 255.255.192.0
/17 is 255.255.128.0
/16 is 255.255.0.0
/15 is 255.254.0.0
/14 is 255.252.0.0
/13 is 255.248.0.0
/12 is 255.240.0.0
/11 is 255.224.0.0
/10 is 255.192.0.0
/9 is 255.128.0.0
/8 is 255.0.0.0
/7 is STOP! no such thing!

So, anyway, on to your point:

/23 uses this mask: 255.255.254.0

Block size is this: 2.0

/22 uses this mask: 255.255.252.0

Block size is this: 4.0

Look at this one:

255.255.255.252, which is a /30

Block size is still 4
0.0
0.4
0.8
0.12
0.16
0.20
0.24
... etc.

Let me know if you still don't understand this explanation.

Block size is 256 - the interesting octet

When ever I get confused, I use binary and do it manually, but having the masks and block sizes memorized helps a lot.

SurferdudeHB

What's the Host/Net 126 65 30 14 6 2 1 0 used for?

jwashington1981

SurferdudeHB wrote: »

What's the Host/Net 126 65 30 14 6 2 1 0 used for?

Not clear on exactly what you're asking there.

instant000

SurferdudeHB wrote: »

What's the Host/Net 126 65 30 14 6 2 1 0 used for?

I think it's supposed to be number of hosts per network, but it is wrong.

Should be this:
mask use: /25 /26 /27 /28 /29 /30
block size: 128 064 032 016 008 004 --- ---
no. hosts: 126 062 030 014 006 002 --- ---

You can't support hosts with a /31 or /32

There is an RFC to allow the usage of a /31 on point-to-point links, but as far as I know, no one implements it.

And, a /32 refers to a specific host, and isn't a network.