IP Address Classes (A, B, C, D, E) – Concepts, Ranges, and Exam Guide

1. Introduction to IP Address Classes

Historically, the IPv4 address space was divided using a classful addressing scheme to simplify routing and address allocation. This system split all 32-bit IPv4 addresses into five classes — A, B, C, D, and E — each identified by the leading bits of the first octet and designed for a distinct purpose.

Classful addressing was the standard from the early 1980s until 1993, when CIDR (Classless Inter-Domain Routing) replaced it. Despite this, a thorough understanding of the classes remains essential for the CCNA exam, for recognising private address ranges, and for troubleshooting legacy network configurations. 

  ​ 0         1         2         3
  ​ 0         0         0         0
  ​ |         |         |         |
  ​ 0xxxxxxx  xxxxxxxx  xxxxxxxx  xxxxxxxx  →  Class A  (0xxx xxxx)
  ​ 10xxxxxx  xxxxxxxx  xxxxxxxx  xxxxxxxx  →  Class B  (10xx xxxx)
  ​ 110xxxxx  xxxxxxxx  xxxxxxxx  xxxxxxxx  →  Class C  (110x xxxx)
  ​ 1110xxxx  xxxxxxxx  xxxxxxxx  xxxxxxxx  →  Class D  (1110 xxxx) Multicast
  ​ 1111xxxx  xxxxxxxx  xxxxxxxx  xxxxxxxx  →  Class E  (1111 xxxx) Experimental

Related pages: IP Addresses | Subnetting & VLSM | Subnetting Basics | Private vs Public IP | NAT & PAT | IPv6 Addressing

2. Class A Addresses

Class A was designed for the world's largest organisations and early Internet backbone providers. Each Class A network block contains over 16 million host addresses.

Property Detail
First octet range 1 – 126
Full address range 1.0.0.0 – 126.255.255.255
Leading bits 0 (first bit of first octet is always 0)
Default subnet mask 255.0.0.0 (/8)
Network / host bits 8 network bits / 24 host bits
Number of networks 126 (27 − 2; 0 and 127 are reserved)
Hosts per network 16,777,214 (224 − 2)
Private range (RFC 1918) 10.0.0.0 – 10.255.255.255 (/8)
Typical use Very large organisations, ISPs, government agencies, cloud VPCs
Example address 10.0.0.1 (commonly used private Class A address)

Note: 127.0.0.0/8 technically falls within the Class A first-octet range but is entirely reserved for loopback. 0.0.0.0/8 is also reserved ("this network"), so the usable network count is 126, not 128.

3. Class B Addresses

Class B was designed for medium-to-large organisations such as universities, government departments, and large enterprises. Each block provides over 65,000 host addresses.

Property Detail
First octet range 128 – 191
Full address range 128.0.0.0 – 191.255.255.255
Leading bits 10 (first two bits are always 10)
Default subnet mask 255.255.0.0 (/16)
Network / host bits 16 network bits / 16 host bits
Number of networks 16,384 (214)
Hosts per network 65,534 (216 − 2)
Private range (RFC 1918) 172.16.0.0 – 172.31.255.255 (/12)
Typical use Universities, large enterprises, regional ISPs
Example address 172.16.0.1 (private Class B address)

4. Class C Addresses

Class C is the most numerous class and was designed for small organisations. Each block supports only 254 hosts, making it well-suited to small offices and home networks. The vast majority of private LAN deployments use Class C addressing (192.168.x.x).

Property Detail
First octet range 192 – 223
Full address range 192.0.0.0 – 223.255.255.255
Leading bits 110 (first three bits are always 110)
Default subnet mask 255.255.255.0 (/24)
Network / host bits 24 network bits / 8 host bits
Number of networks 2,097,152 (221)
Hosts per network 254 (28 − 2)
Private range (RFC 1918) 192.168.0.0 – 192.168.255.255 (/16 block)
Typical use Home networks, small businesses, LANs
Example address 192.168.1.1 (the most common home router address)

5. Class D Addresses – Multicast

Class D addresses are not assigned to individual hosts. Instead, they identify multicast groups — a set of receivers that have joined a group to receive a particular stream of traffic. A single packet sent to a multicast address is delivered to all group members simultaneously.

Property Detail
First octet range 224 – 239
Full address range 224.0.0.0 – 239.255.255.255
Leading bits 1110 (first four bits are always 1110)
Subnet mask N/A — not used for unicast host assignment
Well-known multicast addresses 224.0.0.1 — All hosts on segment
224.0.0.2 — All routers on segment
224.0.0.5 — All OSPF routers
224.0.0.6 — OSPF Designated Routers
224.0.0.9 — All RIPv2 routers
224.0.0.10 — All EIGRP routers
Managed by IGMP (Internet Group Management Protocol) on hosts; PIM on routers

6. Class E Addresses – Experimental

Class E addresses are entirely reserved by IANA and have never been allocated for general use on the public Internet. They were set aside for research, testing, and future protocol development.

Property Detail
First octet range 240 – 255
Full address range 240.0.0.0 – 255.255.255.255
Leading bits 1111 (first four bits are always 1111)
Notable exception 255.255.255.255 — limited broadcast address (used by DHCP Discover)
Status Never deployed in production; not routed on the public Internet

7. Special Addresses and Reserved Ranges

Address / Range Class Purpose Routable?
0.0.0.0/8 A "This network" — used by devices before they have an IP; default route notation No
10.0.0.0/8 A (private) RFC 1918 private range — large enterprise and cloud networks No — NAT required
127.0.0.0/8 A (reserved) Loopback — traffic stays on the local host; 127.0.0.1 is conventional No
169.254.0.0/16 B (link-local) APIPA — self-assigned when DHCP is unavailable; indicates a DHCP failure No
172.16.0.0/12 B (private) RFC 1918 private range — covers 172.16.0.0 through 172.31.255.255 No — NAT required
192.168.0.0/16 C (private) RFC 1918 private range — home routers, small offices No — NAT required
224.0.0.0/4 D Multicast group addressing; managed by IGMP/PIM Multicast only
255.255.255.255 E Limited broadcast — delivered to all hosts on the local segment; not forwarded by routers No

8. Identifying IP Class from the First Octet

On the CCNA exam you will frequently need to determine the class of an address at a glance. The rule is simple — look only at the first octet:

First Octet Value Class Leading Bits Quick Memory Aid
1 – 126 A 0 Below 127 = A (remember: 127 is loopback)
128 – 191 B 10 128–191 = B; halfway through the range
192 – 223 C 110 192+ = C; most home/office addresses live here
224 – 239 D 1110 224+ = D for multicast (routing protocols)
240 – 255 E 1111 240+ = E, experimental, never used in production

Practice example: Is 172.20.5.1 Class A, B, or C?
First octet = 172. Range 128–191 → Class B. It also falls in the RFC 1918 private range (172.16–172.31), so it is a private Class B address.

9. Limitations of Classful Addressing

The classful system was practical when the Internet was small, but it caused severe problems as the network grew:

Problem Explanation Example
Address wastage Allocations were all-or-nothing per class. An organisation needing 300 hosts had to receive a full Class B block (65,534 addresses), wasting over 65,000 addresses. Company needs 300 hosts → assigned 65,534 addresses; 65,234 wasted
Fixed boundaries Subnet masks were fixed per class with no way to subdivide a block to match actual needs. No way to give one department /26 and another /28 from the same block
Routing table bloat Every Class C network appeared as a separate route in global routing tables, causing them to grow unmanageably large. Millions of /24 routes in the global BGP table by early 1990s
IPv4 exhaustion acceleration Waste from large classful blocks hastened depletion of the 4.3 billion IPv4 address space. IANA IPv4 exhaustion in 2011, earlier than necessary

Solution — CIDR (RFC 1519, 1993): Classless Inter-Domain Routing introduced variable-length subnet masks (VLSM), allowing any prefix length (e.g., /19, /27) regardless of the classful boundary. This enabled precise address allocation, route summarisation, and dramatically slowed IPv4 exhaustion. See: Subnetting Basics | Private vs Public IP

10. Full Quick-Reference Summary Table

Class First Octet Leading Bits Default Mask Networks Hosts / Network Private Range (RFC 1918) Purpose
A 1 – 126 0 /8 126 16,777,214 10.0.0.0/8 Very large organisations, ISPs
B 128 – 191 10 /16 16,384 65,534 172.16.0.0/12 Universities, large enterprises
C 192 – 223 110 /24 2,097,152 254 192.168.0.0/16 Small businesses, home networks
D 224 – 239 1110 N/A N/A N/A N/A Multicast groups (OSPF, EIGRP, RIP, IGMP)
E 240 – 255 1111 N/A N/A N/A N/A Experimental / reserved — never deployed

11. Key Points & CCNA Exam Tips

  • Memorise the five class ranges by first octet: A (1–126), B (128–191), C (192–223), D (224–239), E (240–255)
  • The gap at 127 is intentional — the entire 127.0.0.0/8 block is reserved for loopback
  • Memorise the three RFC 1918 private ranges: 10/8, 172.16/12, 192.168/16 — see Private vs Public IP
  • Default masks: Class A = /8, Class B = /16, Class C = /24 — these are classful defaults only; CIDR allows any prefix
  • Hosts per network formula: 2host bits − 2 (subtract network address and broadcast)
  • Class D = multicast only, no subnet mask, no host assignment; managed by IGMP/PIM
  • Class E = experimental, never routed publicly; 255.255.255.255 is the limited broadcast exception
  • CIDR replaced classful addressing in 1993 (RFC 1519) to eliminate waste and enable route summarisation
  • You can identify a class from first octet alone — no need to look at the rest of the address
  • On the exam: if asked about private ranges, always cite RFC 1918

IP Address Classes Quiz

1. What is the default subnet mask for Class A IP addresses?

Correct answer is C. Class A uses a /8 default mask (255.0.0.0), giving 8 network bits and 24 host bits — supporting over 16 million hosts per network.

2. What is the first octet range for Class B IP addresses?

Correct answer is A. Class B addresses have a first octet between 128 and 191, identified by the leading bits 10.

3. Which class of IP address is reserved for multicast?

Correct answer is D. Class D addresses (224.0.0.0 – 239.255.255.255) are used exclusively for multicast group addressing. Well-known examples include 224.0.0.5 (OSPF) and 224.0.0.10 (EIGRP).

4. How many hosts per network does a Class C address support by default?

Correct answer is B. Class C uses a /24 mask leaving 8 host bits. 28 − 2 = 254 usable host addresses per network.

5. Which IP address is reserved for loopback testing?

Correct answer is C. 127.0.0.1 is the loopback address (localhost). Packets sent to it never leave the device and are used to verify the local TCP/IP stack is functional. The entire 127.0.0.0/8 block is reserved for this.

6. Which class IP range is used for private networks within 172.16.0.0 to 172.31.255.255?

Correct answer is A. The 172.16.0.0 – 172.31.255.255 block (172.16.0.0/12) is the RFC 1918 private range belonging to Class B (first octet 128–191).

7. What is the first bit pattern of the first octet for Class C IP addresses?

Correct answer is D. Class C addresses always begin with binary 110 in the first octet, placing the first-octet value in the range 192–223.

8. Why was classful addressing replaced by CIDR?

Correct answer is B. CIDR (RFC 1519, 1993) introduced variable-length subnet masks, allowing any prefix length. This eliminated the all-or-nothing class allocation, dramatically reducing address waste, shrinking global routing tables, and slowing IPv4 exhaustion.

9. What is the purpose of Class E IP addresses?

Correct answer is A. Class E addresses (240.0.0.0 – 255.255.255.255) are reserved by IANA for experimental use. They have never been allocated for production networking. Note that 255.255.255.255 within this range serves as the limited broadcast address.

10. Which of the following is a private IP address range for Class C?

Correct answer is C. The RFC 1918 private range for Class C is 192.168.0.0 – 192.168.255.255 (a /16 block). This is the range used by virtually all home routers and small office networks.

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