Question 1

What is a subnet mask?

Accepted Answer

A subnet mask defines which portion of an IP address identifies the network and which identifies the host. Written as 255.255.255.0 (or /24 in CIDR notation), it determines the size and scope of a subnet. The /24 means the first 24 bits are the network portion.

Question 2

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its network prefix length together, e.g. 192.168.1.0/24. The number after the slash indicates how many bits are the network part. /24 = 256 addresses, /16 = 65,536 addresses.

Question 3

How many usable hosts does a subnet have?

Accepted Answer

Total addresses = 2^(32 − prefix). Subtract 2 for the network address (first) and broadcast address (last). A /24 subnet has 256 total addresses and 254 usable host addresses. A /30 has 4 total and 2 usable — common for point-to-point links.

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