SUBNETTING IPV4

[PietroPoliseno27977]

Hi everyone I'm studying cisco as a self-taught I came to subnetting. By doing an online exercise, the following happened to me:

What are the subnet mask, first available host address of subnet one (NOT subnet zero), and the maximum number of hosts per subnet for the network below?
You need to allow for maximum number of hosts. Also, you can use subnet zero and the all-ones subnet. RFC 1878

Network ID: 177.3.0.0

Subnets Required: 99

The answer is:

Subnet Mask: 255.255.254.0

1st Available Host Address of Subnet 1: 177.3.2.1

Max # of hosts/subnet: 510

Why is 177.3.2.1?

I think it's wrong, however, I rely on you who are more experienced than me

[omz]

Hi

http://www.subnet-calculator.com/subnet.php?net_class=B

[PietroPoliseno27977]

Yes I too have thought so, however in the solution there is 176.3.2.1 as an answer I would have put 176.3.1.1 because it specified when it says: first available host address of subnet one (NOT subnet zero) but not 2.1 because he gave me 2

[omz]

what is the source of your question? If you are using some sort of online practice questions then 

remember - not all online practice questions are correct. 

You might want to ask the course/question author for an explanation why he chooses that answer.

hope this helps

please don't forget to rate helpful posts/solutions

[PietroPoliseno27977]

The site is https://www.nybi.org/subnet-1l.php;


can you tell me some reliable site with correct answers? I have to practice in fact there is another one that instead of giving me 215.59.180.1 mi from 215.59.180.5 when the maximum hosts were 4 and he answers me 5


do you recommend any site?

[omz]

ok .. thanks for sharing the source..

they dont want you to use subnet zero (first subnet)

its picking the 1st IP from the 2nd available subnet

[PietroPoliseno27977]

If you don't select subnet 0 the first available ip is 1.1 and not 2.1 why 2? if you don't consider 0 then there is 1

[omz · Accepted solution]

with /23 (255.255.254.0) subnet mask the 1st subnet id - 177.3.0.0 and broadcast is 172.3.1.255
so the next available subnet is 177.3.2.0/23
177.3.1.0 is covered under 177.3.0.0/23

[PietroPoliseno27977]

So when he says: "Also, you can use subnet zero and the all-ones subnet" does it mean that he uses subnet 0 and 1 a single subnet? But in reality it doesn't happen like this or am I wrong?

[omz]

when they saying subnet-zero its the 1st subnet and all 1s subnet is the last subnet in the range.

for example - 

192.168.0.0/27

will give us 8 subnets 

192.168.0.0/27 << first subnet bits zero

192.168.0.32/27

192.168.0.64/27

192.168.0.96/27

192.168.0.128/27

192.168.0.160/27

192.168.0.192/27

192.168.0.224/27 << last subnet bits 1s

[pigallo]

Hello,

---

@PietroPoliseno27977 wrote:
So when he says: "Also, you can use subnet zero and the all-ones subnet" does it mean that he uses subnet 0 and 1 a single subnet? But in reality it doesn't happen like this or am I wrong?

---

you have to split the question in two parts.

1- What are the subnet mask, first available host address of subnet one (NOT subnet zero)

--->  you already found how to solve this part.

2- the maximum number of hosts per subnet for the network below?
---> The second part has also a note:
You need to allow for maximum number of hosts. Also, you can use subnet zero and the all-ones subnet. RFC 1878.

This note is not related to first part of the question and it doesn't pose any issue/constraint to the number of hosts per subnet which remain 510.
When he says: "Also, you can use subnet zero and the all-ones subnet" means that when you start to address your network you can also take addresses from subnet zero and all ones subnet to maximize amount of hosts used.

In past this practice of excluding subnet zero and all ones subnet from addressing was encouraged but later with time it was declared obsolete.

Best regards.

[omz]

to answer your original question - 

Network ID: 177.3.0.0

Subnets Required: 99

The answer is:

Subnet Mask: 255.255.254.0

1st Available Host Address of Subnet 1: 177.3.2.1

Max # of hosts/subnet: 510

They asked for 99 subnets. 

1st subnet - 177.3.0.0/23 << they want to use the first subnet

2nd subnet - 177.3.2.0/23

177.3.2.1 is the 1st ip in 2nd subnet

BTW - it looks like a pretty good practice site :) I have not come across anything similar