CompTIA Exam Prep - ITF+, A+, Network+, Security+, CySA+

Before CIDR, IPv4 addresses were divided into fixed classes determined by the leading bits of the first octet. Classful addressing has been obsolete in practice since 1993, and yet it is still examined — partly because the vocabulary survives in everyday speech, and partly because understanding why it failed is the fastest route to understanding why CIDR exists. Classful addressing fixed the network-host split at three possible points. CIDR let it fall anywhere, and that flexibility is what saved IPv4. The Five Classes Class First octet Leading bits Default mask Networks Hosts each A 1–126 0 255.0.0.0 (/8) 126 16,777,214 B 128–191 10 255.255.0.0 (/16) 16,384 65,534 C 192–223 110 255.255.255.0 (/24) 2,097,152 254 D 224–239 1110 Multicast — no mask — — E 240–255 1111 Experimental — reserved — — Note the gap: 127 is missing from class A because 127.0.0.0/8 is reserved for loopback. That is why class A runs 1–126 rather than 1–127, and it is a detail exams ask about direct...

RFC 1918 defines the three ranges of IPv4 addresses reserved for private networks. Every home router, every corporate LAN, and every cloud VPC draws from them, and they are among the few things on a networking exam worth memorizing digit for digit. Private addresses are free, reusable by everyone, and never routed on the public internet. The Three Ranges Range CIDR Addresses Old class Typical use 10.0.0.0 – 10.255.255.255 10.0.0.0/8 16,777,216 One class A Large enterprises, cloud VPCs 172.16.0.0 – 172.31.255.255 172.16.0.0/12 1,048,576 16 class Bs Medium networks, Docker defaults 192.168.0.0 – 192.168.255.255 192.168.0.0/16 65,536 256 class Cs Home and small office The 172.16.0.0/12 range is the one people get wrong, so it is worth a moment. It runs from 172. 16 .0.0 to 172. 31 .255.255 — sixteen contiguous /16 blocks. Addresses such as 172.15.x.x and 172.32.x.x are public , and exam questions exploit that boundary deliberately. If you see a 172 address, check the second...