Learning Objectives

• Understand the difference between network, broadcast, and host addresses
• Calculate network and broadcast addresses
• Determine the usable host range

Three Address Types in Every Subnet

Every subnet has three types of addresses:

Network Address

The first address in the subnet. All host bits are 0. This identifies the subnet itself and cannot be assigned to a device.

Example: 192.168.1.0 with a /24 mask.

Broadcast Address

The last address in the subnet. All host bits are 1. This is used to send data to every device on the subnet. It cannot be assigned to a device.

Example: 192.168.1.255 with a /24 mask.

Host Addresses

Everything between the network and broadcast addresses. These can be assigned to devices.

The −2 Rule

For any subnet, the number of usable host addresses is:

Usable hosts = Total addresses − 2

The −2 accounts for the network address and the broadcast address.

This is the single most common mistake in subnetting — forgetting to subtract 2!

Practical Examples

/24 Network (192.168.1.0/24)

| Field | Value | |-------|-------| | Network Address | 192.168.1.0 | | First Usable Host | 192.168.1.1 | | Last Usable Host | 192.168.1.254 | | Broadcast Address | 192.168.1.255 | | Total Addresses | 256 | | Usable Hosts | 254 |

/28 Network (192.168.1.0/28)

| Field | Value | |-------|-------| | Network Address | 192.168.1.0 | | First Usable Host | 192.168.1.1 | | Last Usable Host | 192.168.1.14 | | Broadcast Address | 192.168.1.15 | | Total Addresses | 16 | | Usable Hosts | 14 |

/30 Network (192.168.1.0/30)

This is the typical point-to-point link subnet:

| Field | Value | |-------|-------| | Network Address | 192.168.1.0 | | First Usable Host | 192.168.1.1 | | Last Usable Host | 192.168.1.2 | | Broadcast Address | 192.168.1.3 | | Total Addresses | 4 | | Usable Hosts | 2 |

Special Case: /31

The /31 subnet has only 2 addresses — network and broadcast. But RFC 3021 allows using both addresses as point-to-point links, giving you 2 usable hosts. This is commonly used for router-to-router links.

Key Takeaways

• Network address: all host bits 0 (first address)
• Broadcast address: all host bits 1 (last address)
• Usable hosts = total − 2 (except /31)
• The −2 rule is the #1 subnetting exam mistake

Next lesson: working with /24 subnets.