Analyzing IP Networks – Answers, Exercise 3

By certskills July 17, 2012 07:00

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
Are You Ready to Pass?
By certskills July 17, 2012 07:00
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April 27, 2017 7:00 am

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?

Reply to  Paolo
April 27, 2017 7:35 am

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.

Reply to  lyjo
April 27, 2017 8:51 am

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

May 16, 2017 1:33 pm

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

Reply to  ziad4unix
September 3, 2018 4:13 am

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
Davood Hosseinzadeh
March 19, 2022 12:35 pm

Hi sir,

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


Davood Hosseinzadeh
Davood Hosseinzadeh
Reply to  certskills
April 4, 2022 9:35 am


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
Davood Hosseinzadeh
April 10, 2022 1:28 pm

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?


September 30, 2022 7:31 am

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