ITIS-468-47605: Certified Information Systems Security Professional (CISSP) Cert Exam Prep

Subnetting Refresher

Subnetting divides one network into smaller segments so traffic and trust boundaries can be controlled separately. It is an asset-protection tool as much as a networking one: segmentation limits how far an attacker can move.

• The subnet mask marks which bits are network and which are host.
• Borrowing host bits creates more subnets with fewer hosts each.
• Usable hosts per subnet = 2^(host bits) - 2 (network and broadcast addresses are reserved).
• Number of subnets created = 2^(borrowed bits).
• Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.

Worked example: you are given 192.168.10.0/24 and need at least six subnets. Borrow 3 bits (2^3 = 8 subnets), leaving 5 host bits, so each subnet supports 2^5 - 2 = 30 hosts. The new mask is /27, or 255.255.255.224, and subnets increment by 32: .0, .32, .64, .96, and so on.