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
Telnet Config
By certskills July 17, 2012 07:00
Write a comment


  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
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