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Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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7.4 Subnet masks and CIDR

In this part you’ll revisit subnet masks but in a little more depth than you’ve seen previously. This will reinforce and extend your understanding of how the subnet mask indicates ‘network’ parts of the IP address and ‘host’ parts. You’ll also learn how a process called Classless Inter-Domain Routing (abbreviated as CIDR) achieves more efficient use of available network addresses in the IPv4 addressing scheme.

There are four videos in this part – each video is followed by an activity.

Watch the video below, which is about 3 minutes long.

Revision

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Activity 7 Test yourself

5 minutes

  • Identify the true statement from the options below for the IP address 192.168.2.10 with its subnet mask of 255.255.255.0.

  • a. 

    The host portion of the IP address is 192.168.2.


    b. 

    The IP address 192.168.2.254 is in the same network.


    c. 

    The maximum number of device addresses on this network is less than 10.


    The correct answer is b.

    b. 

    Correct. All IP addresses that start 192.168.2 are in the same network.


Watch the video below, which is about 3 minutes long.

Classful and classless addresses

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Activity 8 Test yourself

5 minutes

  • 1. In the old classful addressing system, if an organisation needed 1500 network addresses, roughly how many network addresses from their allocation block would have been wasted?

  • The organisation would have been given a Class B address block with just over 65,500 addresses as a Class C address block (256 addresses) would have been too small. Therefore about 64,000 addresses would have been wasted.

  • 2. Express the following in CIDR notation:

    • IP address: 192.168.100.0
    • Subnet mast: 255.255.0.0
  • The subnet mask indicates that there are 16 bits in the network address, therefore in CIDR notation the IP address would be expressed as 192.168.100.0/16.

Watch the video below, which is about 2 minutes long.

CIDR

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Activity 9 Test yourself

1 minute

  • What is the size of the host portion of the following IP address (expressed in CIDR notation): 192.168.10.0/24?

  • /24 indicates that there are 24 bits in the network portion of the address. Therefore there must be 8 bits in the host portion.

Watch the video below, which is about 4 minutes long.

Subnetting with CIDR

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Activity 10 Test yourself

5 minutes

Imagine you are a network manager who has been allocated an IP address block of 512 addresses.

  • 1. What would be the CIDR notation for your network?

  • A block of 512 addresses would have 9 bits in the host portion (29 = 512). This would mean that the network portion would be 32 − 9 = 23 bits. Therefore the CIDR notation would be /23.

  • 2. How many subnetworks with 128 addresses would you be able to create?

  • 512/128 = 4, therefore four subnets would be possible.

  • 3. What would be the CIDR notation for each of these four new subnets?

  • 128 addresses in the host portion would require 7 bits (27 = 128), leaving 25 bits in the network portion. Therefore the CIDR notation would be /25.