General Subnetting question

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

Find more posts tagged with

Comments

rob42

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...

RonNewYork

I think I just got it. THANKS!!!!!!!But one more question: how many hosts per network?

rob42

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

rob42

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

RonNewYork

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!

rob42

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.

RonNewYork

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.

RonNewYork

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...?

rob42

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.

RonNewYork

I'll rewrite. SORRY!!!!!!!!!

RonNewYork

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

rob42

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.

RonNewYork

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)

RonNewYork

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.

rob42

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.