Classful networking organizes IPv4 addresses around fixed network boundaries and default prefix lengths. Classes A, B, and C support host assignment; D serves multicast and E experimental purposes. section 7.3.4 section 7.4
Table 7.2 IPv4 address classes (view table figure)
| Class |
First octet bit pattern |
First octet decimal range |
Prefix length |
Note |
|---|---|---|---|---|
| A |
0xxxxxxx |
0–127 |
/8 |
Address range: 0.0.0.0–127.255.255.255 |
| B |
10xxxxxx |
128–191 |
/16 |
Address range: 128.0.0.0–191.255.255.255 |
| C |
110xxxxx |
192–223 |
/24 |
Address range: 192.0.0.0 to 223.255.255.255 |
| D |
1110xxxx |
224–239 |
Reserved for multicast addresses |
|
| E |
1111xxxx |
240–255 |
Reserved for experimental purposes |
Use the class table to identify an address from its first octet and associate it with its default prefix length. section 7.3.4
The fixed boundaries produce these practical differences:
- Class A uses a /8 prefix, leaving 24 host bits; class B uses /16 with 16 host bits; class C uses /24 with 8 host bits. section 7.3.4
- A smaller network portion creates fewer networks with more hosts each, while a larger network portion creates more networks with fewer hosts each. section 7.3.4
- Reserved ranges still apply: 0.0.0.0/8 is reserved, and 127.0.0.0/8 provides loopback addresses such as 127.0.0.1. section 7.3.4
Recognizing a classful block
The address 172.26.0.0 begins with a class B first-octet pattern, so a practice question that omits the original block can assume 172.26.0.0/16 before subnetting it. section 11.4
