CIDR stands for Classless Inter-Domain Routing. It’s a way to specify IP address allocations and routing more efficiently than the older class-based system. The notation 10.0.1.0/24 breaks down into:

• 10.0.1.0: The base IP address for the subnet.
• /24: The subnet mask or prefix length, indicating how many bits are used for the network portion of the address.

Subnet Mask Explanation

The /24 part means that 24 bits out of the total 32 bits in an IPv4 address are allocated to the network portion. Here’s how you can visualize this:

• An IPv4 address is 32 bits long, represented in 4 octets (8 bits each).
• With /24, you’re using the first 24 bits for the network address, leaving 8 bits for host addresses within that network.

Binary Representation:

• 24 bits of 1 for the network part: 11111111.11111111.11111111.00000000
• In decimal, this translates to the subnet mask 255.255.255.0.

Calculating the IP Range:

• Network Address: The base address 10.0.1.0 is the network address for this subnet. No device can be assigned this address; it represents the network itself.
• Usable IP Addresses:
  • With 8 bits for host addressing, there are 2^8 = 256 possible addresses. However, two of these are reserved:
    • Network Address: 10.0.1.0 (the lowest address)
    • Broadcast Address: 10.0.1.255 (the highest address, used for broadcasting to all devices in the subnet)
  
  Thus, the usable IP addresses range from 10.0.1.1 to 10.0.1.254.
  • First Usable IP: 10.0.1.1
  • Last Usable IP: 10.0.1.254

Explanation of the Range:

• 10.0.1.0 to 10.0.1.255: This range includes:
  • 10.0.1.0: The network address, not usable for devices.
  • 10.0.1.1 to 10.0.1.254: 254 usable addresses for devices like computers, servers, or other network equipment.
  • 10.0.1.255: The broadcast address, used to send messages to all devices in this subnet.

Practical Implication:

• When you define a subnet as 10.0.1.0/24 in AWS or any network environment, you’re specifying that you want to work with this particular range of IP addresses. Devices within this subnet can communicate directly with each other without needing to go through a router, assuming they’re configured correctly.
• Any traffic destined for outside this range would be routed according to the route table associated with this subnet, like going through a NAT Gateway for internet access if it’s a private subnet, or directly to the Internet Gateway if it’s a public subnet.