Question 1

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation represents IP networks compactly: IP/prefix-length. /24 means first 24 bits are network, remaining 8 bits are host addresses. /24 = 256 addresses, /25 = 128, /26 = 64. CIDR replaced class-based addressing (A, B, C) for flexible allocation.

Question 2

How do I calculate subnet mask from CIDR?

Accepted Answer

Convert CIDR number to binary: 24 binary ones followed by zeros = 11111111.11111111.11111111.00000000 = 255.255.255.0. Formula: mask = (2^32 - 2^(32-CIDR)). /24 gives 255.255.255.0, /16 gives 255.255.0.0, /8 gives 255.0.0.0. Use calculator for fractional CIDR like /18.

Question 3

What is network address vs broadcast address?

Accepted Answer

Network address: first address in subnet, identifies the network itself (all host bits zero). Broadcast address: last address, used to send to all hosts (all host bits ones). Usable host addresses are between network and broadcast. For /24: network is .0, broadcast is .255, hosts are .1 through .254.

Question 4

How many hosts can fit in a /24 subnet?

Accepted Answer

/24 subnet has 8 host bits: 2^8 = 256 total addresses. Subtract network (.0) and broadcast (.255) = 254 usable hosts. Formula: usable = 2^(32-CIDR) - 2. /25 = 126 hosts, /26 = 62, /27 = 30, /28 = 14, /29 = 6, /30 = 2 (point-to-point), /31 = 0 (special), /32 = 1 (single host).

Question 5

What are private IP ranges?

Accepted Answer

RFC 1918 private ranges: 10.0.0.0/8 (10.x.x.x - 10.255.255.255), 172.16.0.0/12 (172.16.x.x - 172.31.255.255), 192.168.0.0/16 (192.168.x.x). Not routable on public internet, used internally. NAT translates private to public for internet access. Private IPs free, public IPs require allocation.

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