IPv4 Class B Addresses (128.0.0.0 – 191.255.255.255)
Explore IPv4 Class B addresses with /16 default mask. Covers the 128–191 first octet range, binary structure, and use in medium to large organizations.
Detailed Explanation
IPv4 Class B Addresses
Class B addresses use the first two octets for the network ID and the last two for host IDs, providing a balanced address space suitable for medium to large organizations.
Range and Structure
| Property | Value |
|---|---|
| Range | 128.0.0.0 – 191.255.255.255 |
| First octet | 128 – 191 |
| Default Mask | 255.255.0.0 (/16) |
| Networks | 16,384 |
| Hosts per network | 65,534 |
Binary Identifier
Class B addresses always start with 10:
10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
│ │ │
Network (16 bits) Host (16 bits)
Examples
| Address | Network | Host |
|---|---|---|
| 172.16.1.100 | 172.16.0.0 | 0.0.1.100 |
| 150.100.50.25 | 150.100.0.0 | 0.0.50.25 |
| 191.255.0.1 | 191.255.0.0 | 0.0.0.1 |
Private Range Within Class B
The private range 172.16.0.0 – 172.31.255.255 falls within Class B. This provides 16 Class B-sized networks (172.16.0.0/16 through 172.31.0.0/16) for internal use.
Real-World Usage
Universities and medium-sized ISPs were historically assigned Class B blocks. With CIDR, these are now subnetted more efficiently — a /16 can be divided into 256 /24 subnets, each serving a department, floor, or VLAN.
Use Case
A university IT department manages its 150.100.0.0/16 allocation by subnetting into /24 blocks per department: 150.100.1.0/24 for CS, 150.100.2.0/24 for Engineering, and so on.