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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  1. Paolo April 27, 07:00

    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 this comment
    • CCENTSkills April 27, 07:35

      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 this comment
  2. ziad4unix May 16, 13:33

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

    Reply to this comment
    • VidalCastillocz September 3, 04:13

      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.

      Reply to this comment
  3. Davood Hosseinzadeh March 19, 12:35

    Hi sir,

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


    Reply to this comment
    • certskills Author March 29, 14:03

      Hi Davood,
      I wonder if you weren’t aware of the answers appendix C in the back of the book? From the explanation for this question in the appendix:
      The router also adds a route called a local route, which is a route for the interface IP
      address with a mask. In this case, that means the router adds a local
      route for address with mask

      Let me know if that doesn’t answer it for you.

      Reply to this comment
      • Davood Hosseinzadeh April 4, 09:35


        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?

        Reply to this comment
        • certskills Author May 4, 09:06

          CCNA Vol 1, Chap 1, Q#4, the correct answer is B as listed in the book. Not sure why you think answer B is “one computer”, as the text for answer B is “Same-layer interaction”? Same layer interaction occurs in the code that implements the same layer’s functions on two different computers, as described in this question. Hope this helps.

          Reply to this comment
  4. Davood Hosseinzadeh April 10, 13:28

    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?


    Reply to this comment
    • certskills Author May 11, 09:27

      Hey Davood,
      First answer: I’m sure both can be configured, as there is no mechanism to prevent it. The question then is if a default gateway is learned w/ DHCP, does the switch use that value or the configured value? I never tried. I’ve added it to my todo list to try it on a switch or two. I’ll do my best to circle back – may be a few more weeks. If you try it for yourself, let me know.

      Reply to this comment
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