IPv4 Address Classes Explained: A, B, C, D, E for the Network+ Exam

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 directly.

Host counts are always two fewer than the raw total, because the all-zeros address identifies the network and the all-ones address is the broadcast. Neither can be assigned to a host.

Why It Failed

The granularity was hopeless. Suppose an organization needs 500 addresses.

• A class C gives 254 — not enough.
• A class B gives 65,534 — so they get a class B, and 65,034 addresses are wasted.

Multiply that across every organization on a growing internet and the address space drains for no good reason. Meanwhile the global routing table grew unmanageably, because there was no way to summarize many class C networks into one advertisement.

The fix was CIDR — Classless Inter-Domain Routing, RFC 1519, 1993 — which discarded the classes entirely and let the prefix length be any value. That organization needing 500 addresses gets a /23 with 510 hosts, and neighbouring prefixes can be aggregated into a single route. Variable Length Subnet Masking (VLSM) extended the same idea inside an organization, so a point-to-point link gets a /30 while a user VLAN gets a /24.

What Survives

The classes are gone from routing, but the language persists. People still say "a class C network" meaning a /24, and vendor documentation still refers to "class A private space" meaning 10.0.0.0/8. It is loose usage, but you will encounter it and should understand what is meant.

The private ranges of RFC 1918 also map onto the old classes, which is where the habit comes from:

• 10.0.0.0/8 — one class A
• 172.16.0.0/12 — 16 contiguous class Bs
• 192.168.0.0/16 — 256 contiguous class Cs

Recognizing a Class Quickly

Read the first octet and nothing else:

• 1–126 → A
• 127 → loopback, not a usable class
• 128–191 → B
• 192–223 → C
• 224–239 → D, multicast
• 240–255 → E, experimental

The boundaries are easier to recall as powers of two: 128, 192, 224, 240 — each one adds a leading 1 bit.

Network+ (N10-009) Exam Tips

• Memorize the first-octet ranges and the default masks. This is pure recall and it is fast marks.
• Know that 127 is loopback, which is why class A stops at 126.
• Class D is multicast; class E is experimental. Both appear as distractors.
• Understand why CIDR replaced classes — address waste and routing table growth.
• Know CIDR notation and VLSM, which is what modern questions actually test.
• Usable hosts are 2^n − 2. Apply it every time.
• Do not confuse address class with the RFC 1918 private ranges — related, but different concepts.

A+ and Security+ Angles

A+ (220-1201/1202) covers IPv4 addressing basics: public versus private, static versus dynamic, subnet masks, and default gateways. Class terminology may appear but the depth is shallow.

Security+ (SY0-701) cares about subnetting as segmentation — using prefix length to size and isolate network segments — rather than about the historical classes.

Key Takeaways

• A: 1–126, B: 128–191, C: 192–223, D: 224–239 multicast, E: 240–255 experimental.
• 127 is loopback, which is the gap in class A.
• Default masks are /8, /16 and /24 for A, B and C.
• Classful addressing wasted enormous amounts of space because the granularity was so coarse.
• CIDR replaced it in 1993, allowing any prefix length and route aggregation.
• The vocabulary survives even though the routing behaviour does not.