Question 1

What is a subnet mask?

Accepted Answer

A subnet mask is a 32-bit number that divides an IP address into a network portion and a host portion. Written in dotted-decimal notation (e.g., 255.255.255.0), it tells routers which part of an IP address identifies the network and which part identifies individual devices on that network.

Question 2

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its subnet mask as a single string, such as 192.168.1.0/24. The number after the slash indicates how many leading bits are set in the subnet mask — /24 means 24 bits, equivalent to a 255.255.255.0 mask.

Question 3

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

Accepted Answer

The number of usable hosts in a subnet is 2^(32 − prefix length) − 2. The subtracted 2 accounts for the network address and broadcast address, which cannot be assigned to hosts. For example, a /24 subnet has 2^8 − 2 = 254 usable host addresses.

Question 4

What is the difference between /24 and /16 subnets?

Accepted Answer

A /24 subnet (mask 255.255.255.0) has 254 usable host addresses and is the most common size for small LANs. A /16 subnet (mask 255.255.0.0) has 65,534 usable host addresses, making it suitable for larger networks. The smaller the prefix number, the larger the subnet.

Question 5

What are private IP address ranges?

Accepted Answer

The IANA-designated private IPv4 ranges are: 10.0.0.0/8 (10.0.0.0 – 10.255.255.255), 172.16.0.0/12 (172.16.0.0 – 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 – 192.168.255.255). These addresses are not routable on the public internet and are used for internal networks.

Class	Range	Default mask
A	1–126	/8
B	128–191	/16
C	192–223	/24
D	224–239	Multicast
E	240–255	Reserved

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