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What is the proper CIDR prefix notation for a subnet mask of 255.0.0.0?

What is the proper CIDR prefix notation for a subnet mask of 255.0.0.0?

  • /8
  • /16
  • /24
  • /32

The correct answer is /8.

CIDR Prefix Notation and Subnet Masks Explained

CIDR (Classless Inter-Domain Routing) is a method used to specify IP addresses and their associated network prefixes in a more flexible and efficient way than traditional class-based addressing. CIDR uses a prefix notation to define how many bits of the IP address are dedicated to the network portion versus the host portion. The notation /X signifies that the first X bits are part of the network prefix.

In this explanation, we’ll delve deeper into the relationship between subnet masks and CIDR prefix notation, and why a subnet mask of 255.0.0.0 corresponds to a /8 prefix.

What is CIDR Notation?

CIDR is a method of allocating IP addresses that allows for more flexible subnetting. In CIDR notation, an IP address is followed by a forward slash (/) and a number that indicates how many bits in the address represent the network portion. For example, 192.168.0.0/24 means that the first 24 bits of the IP address are used for the network portion, while the remaining bits are for host identification.

  • CIDR Prefix: The number following the slash in CIDR notation specifies how many bits of the IP address are fixed for the network.
  • Subnet Mask: The subnet mask is a 32-bit number that distinguishes the network portion of the IP address from the host portion. Subnet masks are often represented in dotted-decimal notation, such as 255.255.255.0.

Subnet Mask 255.0.0.0 and CIDR /8

A subnet mask of 255.0.0.0 corresponds to a /8 prefix in CIDR notation. Let’s break down how this works:

Understanding the Subnet Mask 255.0.0.0

  • 255.0.0.0 in binary is: 
    11111111.00000000.00000000.00000000
  • The 255 represents eight 1’s in the binary form, indicating that the first 8 bits of the IP address are part of the network portion.
  • The remaining 0.0.0 portion indicates that the remaining 24 bits are for the host portion.

Thus, this subnet mask allocates 8 bits to the network and leaves 24 bits for hosts, meaning the network can have a large number of possible hosts.

CIDR Prefix Notation

In CIDR notation, the number following the slash represents the number of bits in the subnet mask that are set to 1 (representing the network part). In this case, since the first 8 bits are set to 1, the CIDR prefix notation is /8.

Comparison to Other CIDR Notations

  1. /16 (255.255.0.0):
    • A /16 subnet mask allocates 16 bits to the network portion.
    • 255.255.0.0 in binary is 11111111.11111111.00000000.00000000.
    • This allows for 16 bits for the network and 16 bits for hosts, typically used in Class B networks.
  2. /24 (255.255.255.0):
    • A /24 subnet mask allocates 24 bits to the network portion.
    • 255.255.255.0 in binary is 11111111.11111111.11111111.00000000.
    • This provides for 24 bits of network information and 8 bits for hosts, commonly seen in Class C networks.
  3. /32 (255.255.255.255):
    • A /32 subnet mask is the most restrictive, allocating all 32 bits to the network.
    • 255.255.255.255 in binary is 11111111.11111111.11111111.11111111.
    • This is used for a single host, where no bits are left for host addressing.

Conclusion

A subnet mask of 255.0.0.0 corresponds to the CIDR prefix notation /8 because it allocates the first 8 bits of the IP address to the network portion, leaving the remaining 24 bits for hosts. CIDR notation simplifies IP addressing by allowing flexible subnetting, and understanding this relationship is essential for efficient network management.