CCNA - Subnetting question

dave343

[H]ard|Gawd
Joined
Oct 17, 2000
Messages
1,687
I'm working through the CBT Nuggets video's for ICND1 and currently going through the subnetting. Working through practice sites, I'm getting a majority of the questions right, however I ran into one issue that I don't understand how the site is getting there, and even thought maybe they are wrong.(subnetting.net)

The question was: How many Subnets and usable hosts can you have on a 172.26.00/20 network.

Originally I answered 16 Subnets and 4064 hosts, which I now understand is wrong. (I was doing 16x254 taking 2 away from 256 for the subnet and network) The site however says the correct answer is 16 Subnets and 4096 hosts. How can you have 4096 usable hosts, when you can't be using the broadcast? While I can get to 4096 myself, I didn't think those would all be usable hosts, so this really through me off track. Is the site wrong, or maybe the wording used?

Also, this thing about 256 Subnets, why do I keep seeing the number 256 used? I understand that counting subnets you will eventually hit 256, I just don't understand why it's used as in the above question since I thought it only went from 0-255. Even if you write out the chart there is no 256.

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

Thanks in advance for any guidance. Cheers.
 
Last edited:

IdiotInCharge

[H]F Junkie
Joined
Jun 13, 2003
Messages
14,305
I just don't understand why it's used as in the above question since I thought it only went from 0-255.
Not versed enough in Cisco-speak or really fluent in subnetting these days, but I'll address this: remember to count '0' :). A range of 0-255 means 256 discrete units.
 

Nicklebon

Gawd
Joined
May 22, 2006
Messages
648
What size subnets? Are you trying to maximize hosts or subnets? If your looking for max subnets you could peel out 1024 /30s with 2 hosts each.

As for the book answer 16x /24s at 254 host per network is indeed 4064 hosts.

edit ... Sorry I reread your post. Your answer is correct for 16 subnets not the book's. There are indeed 4064 usable addresses in in 16 /24s networks. However, since a subnet size was not specified I'm really not sure there is a single correct answer. As stated ealier you could also get 1024 /30s with 2048 usable addresses or just as correctly 512 /29s with 3072 usable addresses.
 
Last edited:

dave343

[H]ard|Gawd
Joined
Oct 17, 2000
Messages
1,687
What size subnets? Are you trying to maximize hosts or subnets? If your looking for max subnets you could peel out 1024 /30s with 2 hosts each.

As for the book answer 16x /24s at 254 host per network is indeed 4064 hosts.
Thanks for the reply. It was just a question on a subnetting practice page, however from my understand a network of 172.16.0.0 /20 would have 16 subnets and 4094 *usable* hosts. (4096 -2)... the website is saying the answer is 4096 usable. How can you have 4096 usable since we can't use the broadcast address right. Maybe they accidentally used the word usable and meant general, or am I not looking at it right?
 

ZeqOBpf6

Gawd
Joined
Aug 24, 2014
Messages
617
They forgot to subtract them, or they're just counting them, you're good. Not sure where you saw this question but consider alerting them.
 

Cmustang87

Supreme [H]ardness
Joined
Oct 4, 2007
Messages
4,461
I think in these books and material, they ask it in the form of "how many IPs for host are there" and the correct answer is technically whatever it is before the minus 2. If they specify "usable hosts" then it would be n-2. If you are early in the CCNA course, they want you to understand and quickly calculate host and network bits, rather than what's actually usable - unless specified.

I don't think we can fairly say you are correct or incorrect without seeing the actual question - but you are indeed correct on the usable amount of hosts for a /20 network being 4094.

Edit: forgot to answer this question

Also, this thing about 256 Subnets, why do I keep seeing the number 256 used? I understand that counting subnets you will eventually hit 256, I just don't understand why it's used as in the above question since I thought it only went from 0-255. Even if you write out the chart there is no 256.

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

Thanks in advance for any guidance. Cheers.
In computers we count 0. So, you can have all 0s or all 1s. In ipv4, each octet is 8 bits, representing 256 total numerical values... however, we start at 0, not 1.
 
Last edited:

Nicklebon

Gawd
Joined
May 22, 2006
Messages
648
Thanks for the reply. It was just a question on a subnetting practice page, however from my understand a network of 172.16.0.0 /20 would have 16 subnets and 4094 *usable* hosts. (4096 -2)... the website is saying the answer is 4096 usable. How can you have 4096 usable since we can't use the broadcast address right. Maybe they accidentally used the word usable and meant general, or am I not looking at it right?
Let's be clear:

1. The 172.16.0.0/20 has 4096 IPs. One of these is the network address 172.16.0.0 and the other is 172.16.255.255 the broadcast leaving 4094 host addresses if used as a /20.
2. If you subnet the above address into /24s you will have 16 networks and 4064 host address as you will loose 2 addresses for each subnet leaving 254 host addresses 16*254=4064. Your first answer is the correct answer if you subnet into /24s.
3. Absent /31 links there is no circumstance that network and broadcast addresses are considered host IPs and anyone saying otherwise is simply wrong.
 
Top