Question 1

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation is a compact way to represent an IP address and its associated network mask. It is written as an IP address followed by a forward slash and a number (e.g., 192.168.1.0/24), where the number indicates how many leading bits of the address define the network portion. A /24 means the first 24 bits are the network address and the remaining 8 bits are available for host addresses, giving 256 total addresses (254 usable hosts). CIDR replaced the older classful addressing system to allow more flexible allocation of IP address space.

Question 2

How does subnetting work?

Accepted Answer

Subnetting divides a larger network into smaller, more manageable segments called subnets. Each subnet has its own network address, broadcast address, and range of usable host addresses. By borrowing bits from the host portion of an IP address and adding them to the network portion, you create additional subnets with fewer hosts each. For example, splitting a /24 network into two /25 subnets gives you two networks of 126 usable hosts each instead of one network with 254 usable hosts. Subnetting improves security, reduces broadcast traffic, and makes IP address management more efficient.

Question 3

What is a netmask and how do I convert it to CIDR?

Accepted Answer

A netmask (or subnet mask) is a 32-bit number that separates the network portion of an IP address from the host portion. It is typically written in dotted decimal format, such as 255.255.255.0. To convert a netmask to CIDR notation, count the number of consecutive 1-bits in the binary representation of the mask. For example, 255.255.255.0 is 11111111.11111111.11111111.00000000 in binary, which has 24 ones, so it equals /24. Common netmasks include 255.255.255.0 (/24), 255.255.0.0 (/16), and 255.0.0.0 (/8).

Question 4

What is the difference between /24 and /32?

Accepted Answer

A /24 subnet contains 256 IP addresses (254 usable hosts) and represents a common network allocation for small to medium networks. The network address uses the first 24 bits, leaving 8 bits for host addresses. A /32 represents a single IP address with no host bits, often used in routing tables to specify an exact host route or in access control lists to match one specific IP. The smaller the CIDR number, the larger the network: /16 contains 65,536 addresses, /8 contains over 16 million, and /0 represents the entire IPv4 address space.

Question 5

What are private IP address ranges?

Accepted Answer

Private IP address ranges are reserved by RFC 1918 for use within local networks and are not routable on the public internet. The three private ranges are 10.0.0.0/8 (10.0.0.0 to 10.255.255.255, over 16 million addresses), 172.16.0.0/12 (172.16.0.0 to 172.31.255.255, about 1 million addresses), and 192.168.0.0/16 (192.168.0.0 to 192.168.255.255, about 65,000 addresses). Home routers typically use 192.168.0.0/24 or 192.168.1.0/24, while large enterprises often use the 10.0.0.0/8 range. Network Address Translation (NAT) allows devices with private IPs to access the internet through a single public IP.

Question 6

How do I calculate the number of hosts in a subnet?

Accepted Answer

To calculate the number of usable hosts in a subnet, use the formula 2^(32-prefix) - 2 for IPv4. The subtraction of 2 accounts for the network address (first address, used to identify the subnet) and the broadcast address (last address, used to send to all hosts). For example, a /24 subnet has 2^8 - 2 = 254 usable hosts, a /28 has 2^4 - 2 = 14 usable hosts, and a /30 has 2^2 - 2 = 2 usable hosts (commonly used for point-to-point links). A /31 is a special case defined in RFC 3021 that allows 2 usable addresses with no network or broadcast address, used for point-to-point links.

CIDR / Subnet Calculator

Understanding CIDR Notation

Common Subnet Sizes

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