General Subnetting question

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I'm working through a tutorial (video). The tutorial has gone through a few subnetting examples and I feel I get it. I know binary numbers, etc. rather well. OK the question was:divide 20.0.0.0/8 into 500 networks.I get that the new subnet mask will be /17 as 9 bits will be needed for the subnets (2^9 is 512).BUT then....the first subnetwork would start at 20.0.0.0 and end at 20.0.128.255, but why?Also, the video says there are 2^13-2 host addresses looking at the /17 mask:11111111.11111111.10000000.00000000BUT to me that leaves 7+8 bits for hosts ,which is 15, not 13. Why the 13? And at this point I figure I'm missing something.Any help is appreciated.RON

• Member Posts: 423
Not quite...

1st Subnet
Network 20.0.0.0 /17
B/Cast 20.0.127.255

2nd Subnet
Network 20.0.128.0 /17
B/Cast 20.0.255.255

etc...
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• Member Posts: 16 ■□□□□□□□□□
I think I just got it. THANKS!!!!!!!But one more question: how many hosts per network?
• Member Posts: 423
You're welcome.

For readers that can't see it...

If you 'set' all the 'host bits' (in RED), you get the Broadcast IP Address for the Network.
```IP Address.. 0 0 0 1 0 1 0 0 | 0 0 0 0 0 0 0 0 | 0 [COLOR=#ff0000]1 1 1 1 1 1 1[/COLOR] | [COLOR=#ff0000]1 1 1 1 1 1 1 1[/COLOR] = 20.0.127.255
Mask........ 1 1 1 1 1 1 1 1 | 1 1 1 1 1 1 1 1 | 1 0 0 0 0 0 0 0 | 0 0 0 0 0 0 0 0 = 255.255.128.0
'AND'....... 0 0 0 1 0 1 0 0 | 0 0 0 0 0 0 0 0 | 0 0 0 0 0 0 0 0 | 0 0 0 0 0 0 0 0 = 20.0.0.0
```
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• Member Posts: 423
RonNewYork wrote: »
I think I just got it. THANKS!!!!!!!But one more question: how many hosts per network?

Two to the power of the number of 'Host Bits', less one for the Network and one for the Broadcast...

2^15 = 32768 - 2 = 32766
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• Member Posts: 16 ■□□□□□□□□□
rob42 wrote: »
Two to the power of the number of 'Host Bits', less one for the Network and one for the Broadcast...2^15 = 32768 - 2 = 32766
That's what I thought. The video I was watching said 13 instead of 15 and wrote it down (on the video), and then said 8192-2 (for network and broadcast). That's what confused me. THANKS!
• Member Posts: 423
RonNewYork wrote: »
That's what I thought. The video I was watching said 13 instead of 15 and wrote it down (on the video), and then said 8192-2 (for network and broadcast). That's what confused me. THANKS!

It sounds like you need a new source for video lessons

You're welcome. All the best with your studies.
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• Member Posts: 16 ■□□□□□□□□□
rob42 wrote: »
It sounds like you need a new source for video lessons You're welcome. All the best with your studies.
You might be correct. I posted this on the forum (the other company's) to see what the response is.Maybe there's a correction page that I didn't see. Don't know.
• Member Posts: 16 ■□□□□□□□□□
If no one minds, I have another subnetting question:If the problem is: Divide this network into networks of 60 hosts:200.200.200.0/24Since 60 hosts required 6 bits, we take these 6 bits from the last octet and so end up with two bits for networks, which yields 4 networks with 64 possible hosts on each (minus 1 for the network address and one for the broadcast address).So in CIDR the network would be 200.200.200.0/26so the first network would be:200.200.200.0 to 200.200.200.63and the next would be200.200.200.64 to 200.200.200.127200.200.200.128 to 200.200.200.192200.200.200.192 to 200.200.200.255Correct? Or...?
• Member Posts: 423
No.

First off you'll need a mask of /26 (255.255.255.192) -- 4 Subnets, 62 Hosts per.

1st. 200.200.200.0
2nd. 200.200.200.64
3rd. 200.200.200.128

etc...

Edit: Opps... I can see you've got the Mask (/26) correct.

Dude, it's very hard to read your posts, the way you format them...
...so, yes you're bang on.
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• Member Posts: 16 ■□□□□□□□□□
I'll rewrite. SORRY!!!!!!!!!
• Member Posts: 16 ■□□□□□□□□□
Is there a setting so hard returns are not parsed out? I'm using Chrome 55.
• Member Posts: 423
I'll rewrite. SORRY!!!!!!!!!

Not a worry...
Is there a setting so hard returns are not parsed out? I'm using Chrome 55.

I've no idea; I'm a Firefox user (I can't stand Chrome). I'm sure a fellow Chrome user will be able to help.

Anyways, from what I can make out (not your fault if Chrome is not playing nice), you're good with your Subnets.
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• Member Posts: 16 ■□□□□□□□□□
Just realized: I have a Chrome plug in that blocks Javascript (mainly for some sites which use it annoyingly) and it defaults to Block so it was blocking javascript when posting here.

I think it's fixed now.

(hope so)
• Member Posts: 16 ■□□□□□□□□□
rob42 wrote: »
It sounds like you need a new source for video lessons

You're welcome. All the best with your studies.

I was on howtonetwork.com . I joined because they have a lot of Cisco-related videos and also you can reserve rank time on real racks for the cost of joining (which is like \$20 per month). I don't know anywhere else I can do this. I've tried use GNS3 but it was just an incredible pain to set up. And anyways, I would still need a real Cisco IOS which I don't have.

I'm not having a good time on that network, however. Not sure why.
• Member Posts: 423
RonNewYork wrote: »
I was on ##########.com . I joined because they have a lot of Cisco-related videos and also you can reserve rank time on real racks for the cost of joining (which is like \$20 per month). I don't know anywhere else I can do this. I've tried use GNS3 but it was just an incredible pain to set up. And anyways, I would still need a real Cisco IOS which I don't have.

I'm not having a good time on that network, however. Not sure why.

It's sounds like a bit of a rip-off to me, dude... Mind, that's just my H.O.

I tried GNS3 also, but came the the same conclusion as you, but I'm sure it's fab if you can get it up-and-running; many folks on here recommend it.

For CCNA, CPT is just fine (or so I understand from the posts I read on here), but I have to admit that I like to have 'real' equipment. I found a couple of 2950 switches and a 2600 router on ebay for not too many ££££. If I'm honest, the 2600 was a waste of money, but I also found a C880. It's so nice! No fan (so no noise) and it's running IOS 15.

Have you seen the 'For Sale' posts on this site?

P.s. I glad to see that you've sorted out your text formatting issue.
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