An IP address with a/18prefix (CIDR notation) indicates 18 network bits in the subnet mask, leaving 14 host bits (32 total bits - 18). For IPv4 (e.g., 192.168.0.1):
Binary Mask:First 18 bits are 1s, rest 0s.
1st octet: 11111111 (255)
2nd octet: 11111111 (255)
3rd octet: 11000000 (192)
4th octet: 00000000 (0)
Decimal:255.255.192.0
Calculation:
Bits: /18 = 2^14 hosts (16,384), minus 2 (network/broadcast) = 16,382 usable.
Range: 192.168.0.0–192.168.63.255 (3rd octet: 0–63, as 192 = 11000000 covers 6 bits).
Technical Details:
Subnet masks align on octet boundaries or mid-octet (e.g., 192 = 2^7 + 2^6).
Contrast: /24 = 255.255.255.0 (256 hosts), /16 = 255.255.0.0 (65,536 hosts).
Security Implications:Larger subnets (e.g., /18) increase broadcast domains, risking amplification attacks. CNSP likely teaches subnetting for segmentation (e.g., VLANs).
Why other options are incorrect:
A. 255.255.255.0:/24 (8 host bits), not /18.
B. 255.225.225.0:Invalid mask (225 = 11100001, non-contiguous 1s).
D. 255.225.192.0:Invalid (225 breaks binary sequence).
Real-World Context:Subnetting 192.168.0.0/18 isolates departments in enterprise networks.References:CNSP Official Documentation (IP Addressing); RFC 1918 (Private IP).
