A Class B IPv4 address begins with a first octet from 128 through 191. Its classful default prefix length is /16, leaving 16 bits for the host portion. section 7.3.4 section 7.4
Recognize Class B addresses by these characteristics:
- Binary addresses begin with 10, which produces first-octet values from 128 through 191. section 7.3.4
- The default subnet mask is 255.255.0.0, equivalent to /16. section 7.3.4
- Classful calculations provide 16,384 networks and 65,536 addresses per network before excluding reserved network and broadcast addresses. section 7.3.4
- Classful networking is obsolete; modern classless networking permits more flexible prefix lengths. section 7.4
Subnetting example
For 172.26.0.0 with mask 255.255.252.0, identify 172 as a Class B first octet and assume the original classful block is 172.26.0.0/16 before calculating subnets and usable hosts. section 11.4
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 |
Compare Class B’s bit pattern, decimal range, prefix length, and address range with the other IPv4 address classes. section 7.3.4
