1.2 IPv4 Addressing, CIDR Notation & Subnetting Math

🎯 Key CompTIA Network+ Exam Takeaways

• Subnet masks divide IP addresses into Network and Host portions; usable hosts equals 2^(host bits) - 2.
• Block size (increment) is calculated as 256 minus the interesting octet subnet mask value.
• RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 (up to 172.31.255.255), and 192.168.0.0/16 for non-routable private networks.

IPv4 addresses consist of 32 bits arranged in four 8-bit octets. In modern Classless Inter-Domain Routing (CIDR), the subnet mask dictates how many bits represent the network prefix and how many identify individual hosts.

A /27 subnet mask borrows 3 bits in the 4th octet (255.255.255.224), leaving 5 host bits. This yields 2^5 = 32 total addresses, minus the network identifier and the broadcast address, resulting in exactly 30 usable host IP addresses.

Standard point-to-point links between two routers use /30 prefixes (255.255.255.252), which provide exactly 2 usable host addresses with zero address wastage.

⚠️ Common Pearson VUE Exam Traps

• Remember to subtract 2 for the Network ID and Broadcast address when calculating usable host capacity.
• Do not assume 172.32.x.x is private; the Class B private space ends strictly at 172.31.255.255.