Question 1

What is CIDR notation and how do I read it?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its subnet mask together, e.g. 192.168.1.0/24. The number after the slash is the prefix length — the count of consecutive 1-bits in the subnet mask from the left. /24 means 255.255.255.0, giving 256 addresses (254 usable hosts). /16 is 255.255.0.0 (65,534 usable hosts). /32 is a single host address.

Question 2

What is the difference between a subnet mask and a CIDR prefix length?

Accepted Answer

They convey the same information in different formats. A subnet mask is a 32-bit number written in dotted-decimal (e.g. 255.255.255.0). A CIDR prefix length is the count of 1-bits in that mask (e.g. /24). They are interchangeable: /24 = 255.255.255.0, /16 = 255.255.0.0, /8 = 255.0.0.0. CIDR notation is more compact and modern; subnet masks are used in older network equipment and Windows network settings.

Question 3

What are private (RFC 1918) IPv4 address ranges?

Accepted Answer

RFC 1918 defines three private address ranges that are not routed on the public internet: 10.0.0.0/8 (10.0.0.0–10.255.255.255, ~16.7M addresses), 172.16.0.0/12 (172.16.0.0–172.31.255.255, ~1M addresses), and 192.168.0.0/16 (192.168.0.0–192.168.255.255, ~65,500 addresses). These are used for home networks, corporate LANs, VPNs, and container networking (Docker, Kubernetes).

Question 4

How do I calculate the number of usable host addresses in a subnet?

Accepted Answer

For a subnet with prefix length n, the total number of addresses is 2^(32-n). Subtract 2 for the network address (first) and broadcast address (last) to get usable hosts: usable = 2^(32-n) - 2. Examples: /24 = 254 hosts, /25 = 126 hosts, /26 = 62 hosts, /27 = 30 hosts, /28 = 14 hosts, /29 = 6 hosts, /30 = 2 hosts (point-to-point links), /31 = 2 hosts (special case per RFC 3021), /32 = 1 host (loopback/self).

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