Classful Subnetting | CompTIA Network+ N10-007 | 1.4b

In this video you will learn about classful subnetting concepts such as: classful IP addressing, subnetting, subnet masks, & broadcast IP addresses.

Classful IP Addressing

Cassful IP addressing is a method that divides the IP address space for IPv4 into 5 address classes based on the leading four address bits.  Classes A, B, & C provide unicast addresses for networks of 3 different network sizes.  Class D is for multicast networking and the class E address range is used for future or experimental purposes.

An IP address is divided into 2 fields:  a network prefix (network ID) and a host ID.  What separates the network ID from the host ID depends on whether the address is a Class A, B or C address.

Parts of an IP Address Example
  • Class A:  1 – 126 (1.0.0.1 to 126.255.255.254 IP range) 
    • Supports 16 million hosts on each of 127 networks
  • Class B:  128 – 191 (128.1.0.1 to 191.255.255.254)
    • Supports 65,000 hosts on each of 16,000 networks
  • Class C:  192 – 223 (192.0.1.1 to 223.255.254.254)
    • Supports 254 hosts on each of 2 million networks
  • Class D:  224 – 239 (224.0.0.0 to 239.255.255.255)
    • Reserved for multicast groups
  • Class E:  240 – 255 (240.0.0.0 to 254.255.255.254)
    • Reserved for future use, or research & development purposes

Subnetting

Subnetting is the strategy used to partition a single physical network into more than one smaller logical subnetwork (subnets).  Subnetting allows for an organization to subdivide a large network into smaller, more efficient subnetworks to help minimize traffic and increase network speeds.  Subnets are designed by accepting bits from the IP address’s host part and using these bits to assign a number of small subnetworks inside the original network.  Subnetting allows an organization to add subnetworks without the need to acquire a new network number from an Internet Service Provider (ISP).

Benefits of subnetting:

  • Reduces the network traffic by reducing the volume of broadcasts.
  • Helps to surpass the constraints in the LAN, for example, the maximum number of permitted hosts.
  • Enables users to access a work network from their homes; there is no need to open the complete network.
Subnetting

Subnet Mask

A subnet mask is a 32-bit number created by setting network bits to all 1s and setting host bits to all 0s.  The subnet mask splits the IP address into the network and host addresses, thereby defining which part of the IP address belongs to the network and which part belongs to the host.

Subnet Mask

Subnet Mask Classes

  • Class A:  255.0.0.0
    • 8 bits available for network portion [255] 
    • 24 bits available for host portion [0.0.0]
  • Class B:  255.255.0.0
    • 16 bits for network portion
    • 16 bits for host portion
  • Class C:  255.255.255.0
    • 24 bits for network
    • 8 bits for host
  • Class D & E:  not defined

Put It All Together

  • Class A:  1 – 126 (1.0.0.1 to 126.255.255.254 IP range) 
    • Supports 16 million hosts (16,777,214) on each of 127 networks
    • Subnet Mask:  255.0.0.0
      • 8 bits available for network portion [255]
        • 11111111.00000000.00000000.00000000 
      • 24 bits available for host portion [0.0.0]
        • 00000000.11111111.11111111.11111111
  • Class B:  128 – 191 (128.1.0.1 to 191.255.255.254)
    • Supports 65,000 hosts (65,534) on each of 16,000 networks
    • Subnet Mask:  255.255.0.0
      • 16 bits for network portion [255.255]
        • 11111111.11111111.00000000.00000000
      • 16 bits for host portion [0.0]
        • 00000000.00000000.11111111.11111111
  • Class C:  192 – 223 (192.0.1.1 to 223.255.254.254)
    • Supports 254 hosts on each of 2 million networks
    • Subnet Mask:  255.255.255.0
      • 24 bits for network [255.255.255]
        • 11111111.11111111.11111111.00000000
      • 8 bits for host [.0]
        • 00000000.00000000.00000000.11111111

Learn how to convert from decimal to binary HERE.

Broadcast IP Address

A broadcast address is a special IP address used to transmit messages and data packets to network systems.  Network admins verify successful data packet transmission via broadcast addresses.  Broadcast addresses are also the last IP address within a network ending with .255.  Another way to comprehend what a broadcast IP address is as follows:

  • A host of a competition is preparing to announce the winner of the competition and can use one of the following approaches:
    • The host can walk to each contestant individually and quietly reveal the winner’s name, or
    • The host can announce the winner’s name to all of the contestants and then ask the winner to stand for recognition.
  • The second option is more efficient and is essentially how broadcast addressing works in the real world.

Classful Subnetting Example

Classful Subnetting Example