Q & A for June 29, 2001

Welcome to Certifiable, your exam prep headquarters. Here you'll find questions about some of the tricky areas that are fair game for the certification exams. Following the questions, you'll find the correct answers and explanatory text. We change the questions weekly.

Questions (June 29, 2001)
Answers (June 29, 2001)

A URL Ward Ralston sent me inspired this week's questions. LearnToSubnet.com is a great site for learning TCP/IP subnetting.

A recruiter once asked me what I considered to be the bare minimum skills for a network administrator and/or a programmer. My answer was "Both must know TCP/IP." A good understanding of how TCP/IP addressing works is an absolute requirement for tasks ranging from setting up client computers to routing to network design. This skill is one that you must have if you plan to manage a network in today's world, but it's probably the weakest skill for network administrators—both new and experienced (I know very experienced administrators who still struggle with subnetting their networks).

Programmers also need a solid understanding of TCP/IP addressing because most applications interact in some way with multiple clients across the network. Often an application won't work correctly because something is wrong with IP addressing or routing. Knowing how to troubleshoot those problems without relying on overworked network administrators often means faster responses to customers' needs. Whether you know enough to be a Cisco router administrator is up to you, but a good understanding of TCP/IP will improve all aspects of your application.

If you jump over to the LearnToSubnet site for a quick refresher, I think you'll have no problems with either this week's questions.

Questions (June 29, 2001)

Question 1
You've been assigned a network ID of 192.1.2.0. Which subnet mask will give you 254 hosts per subnet?

  1. 255.255.255.0
  2. 255.248.0.0
  3. 255.255.248.0
  4. 255.255.255.1

Question 2
What is the default subnet mask for a host IP address of 140.150.162.175?

  1. 255.254.0.0
  2. 255.255.255.0
  3. 255.255.0.0
  4. 255.255.255.255

Question 3
Russell Industries has a network address of 160.112.0.0. It needs 36 subnets with a minimum of 600 nodes per subnet. Which of the subnet masks below meets these requirements?

  1. 255.255.192.0
  2. 255.255.255.0
  3. 255.255.248.0
  4. 255.255.252.0

Answers (June 29, 2001)

Answer to Question 1
The correct answer is A—255.255.255.0. The subnet mask 255.255.255.0 is the default subnet mask for a Class C address. Class C addresses have 2,097,152 (i.e., 221) networks available and can have 254 (i.e., 28 - 2) hosts per network. We don't use network address classes as much as we used to because of the inefficiency of assigning whole classes to one entity out of the depleted pool of addresses. However, because most network addresses that ISPs assign are subnetworks of a larger network address class, it's still useful to remember the details about the original classes.

Answer to Question 2
The correct answer is C—255.255.0.0. The 140.150.162.175 address must be recognized as a Class B address because the first octet is in the range of 128 to 191; therefore the subnet mask is 255.255.0.0.

Answer to Question 3
The correct answer is D—255.255.252.0, Because the company needs a minimum of 600 nodes on each subnet, it needs 10 bits for the host part of the IP address (210 = 1024). Because the host ID can't be all 1s or all 0s, you can have 1022 hosts per subnet.

You then have 22 bits available for the network address (32 - 10 = 22). The 160.112.0.0 network address is a Class B address; therefore, the first 16 bits of the subnet mask must be 1s. That leaves 6 bits (22 - 16 = 6) to identify the subnet, which gives you 62 (26 - 2) possible subnets with a maximum of 1022 hosts on each subnet.

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