Analyzing IP Networks – Answers, Exercise 3

 In 200-301 V1 Ch12: Classful IPv4 Networks, 200-301 V1 Part 4: IPv4 Addressing, CCENT-OLD, IPv4 Ch. 12 Network Analysis

Today’s post lists the answers to the previous post’s question, with a few comments, with a place to discuss. Nothing snazzy, but it does hit the fundamentals. Enjoy!

Related links:

Class, Network ID, and Network Broadcast

The Network ID can be derived from the class A, B, or C DDN value by copying the network octets, and writing a 0 for the rest of the octets. Similarly, the network broadcast address can be found by using the same logic, but writing a 255 instead of 0 for the host octets. Table 2 shows the class for each of the five problems, along with the derived network ID and network broadcast address for each class A, B, or C address.

Table 2: Network IDs and Network Broadcast Addresses

DDN Value Class Network ID Network Broadcast Address
1 A
2 C
3 C
4 A
5 123.321.123.321 N/A N/A N/A

Usable Host IP Addresses

To find the range of IP addresses that can be used by hosts in the (unsubnetted) classful network, just add 1 to the network ID and subtract 1 from the network broadcast address. Table 3 shows the results for these five problems.

Table 3: Ranges of Usable Addresses

Network ID Lowest Usable Host Address Highest Usable Host Address Network Broadcast Address
5 N/A N/A N/A N/A
Analyzing IP Networks Exercise 3
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Hi Wendell,

why 123.321.123.321 is marked an N/A?
It seems to me a valid Class A IP address, am i wrong?


Hi Paolo,
It’s easy to overlook, but… check out the 2nd and 4th octet values of “321”. That value is not allowed; it must be between 0-255 inclusive.


Ahahah, shame on me!
I focused only on the first octet.
Thank you for the reply!


aha that last one got me. Careless mistake! Thanks for the practice!


Thanks for pointing this out. The last address looked also valid to me until you specified the range 0-255. Easy to overlook, we could find out later when the value exceed the number in the network mask.

Davood Hosseinzadeh

Hi sir,

In your CCNA-Volume 1, page 368, Q# 2, why one of the correct answers is D and not E?


Davood Hosseinzadeh


CCNA volume 1, chapter 1, Q# 4, answer is B(on one computer), but in page 22, you’ve mentioned ” two computers. What’s the correct answer?

Davood Hosseinzadeh

What will occur to the default-gateway of a SW, if the networker configure both “ip address dhcp” & “ip default-gateway”? Can he configure both?



Hello Mr Odom
in the page 325 Chapter 14: Analyzing Existing Subnets, you wrote:
“The original purpose for the subnet broadcast address was to give hosts a way to send one packet to all hosts in a subnet and to do so efficiently. For example, a host in subnet A could send a packet with a destination address of subnet B’s subnet broadcast address. The routers would forward this one packet just like a packet sent to a host in subnet B. After the packet arrives at the router connected to subnet B, — that last router would then forward the packet to * **all hosts in subnet B***, typically by encapsulating the packet in a data-link layer broadcast frame. As a result, all hosts in host B’s subnet would receive a copy of the packet”
please can you explain me ??? i thought the flooding frames is done only by switches and the routers never broadcast a frame????
and how a router broadcast a packet to all hosts
thank you very much


Hello Wendell,

This question is not related to this exercise, it is a subnetting question I encountered on PTP. This is the question id: Question Id : 200301-GB-v1-P-05-19 and
This is the explanation to the answer: R1, R2, and R3 have IP addresses on their common subnet that are all in the same subnet –, range – As a result, given the assumption that no layer 1 or layer 2 problems exist, all three routers should be able to ping each other. Assigning R4’s Fa0/0 the would put R4 in a different subnet, namely, so R4 would not be able to ping R2’s Fa0/0 IP address. On R2, the attempt to configure the Fa0/1 interface with IP address would be rejected because the subnet (, range – overlaps with the subnet connected to R2’s Fa0/0 interface.

I have underlined the part I need help with. My question is this: How can I calculate/derive the subnet given some ip addresses & masks? In this question, 3 ip addresses were given with /27 masks, I was unable to determine the subnet to which they belong which in this case as per the answer is /19. Hopefully my question makes sense.

Also, how do I use the question id to find the topic to which the question relates? I interpreted this question as coming from volume 1, part 5 and chapter 19. But chapter 19 is not in part 5 of the volume 1 study material.


Hello again Wendell,
Never mind, I see my mistake, I thought the interesting octet was in the last octet. But I still need help figuring out the relevant chapter to a question. Thank you for your time and patience.

Wendell Odom

Hi Tewa,
No worries on the questions. Getting confused and then overcoming that confusion is part of learning. This is a safe place to live out that struggle!
As for figuring out the book content related to an individual question, the app does not supply that information if using the standard product. If you buy the premium edition, which includes PDFs of the books, the app then gives you a link to the related book content, so you can click and go to the PDF pages in that section. However, after reviewing your question, maybe it would be useful to at least display the chapter and heading for all users. So I’m going to ask.
FYI, this question requires knowledge from many chapters. It’s also assigned to the Part Review bank for Part 5. (Looks like you were attempting to interpret the QID.) That 19 in the QID was just a relative number, rather than a chapter. There’s more relevant content in the ping topics in chapter 18. Hope that helps for this question.

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