What are the four key addresses in any subnet and how are they calculated?

Question

Accepted Answer

Network address (host bits all 0), first host (network+1), last host (broadcast−1), and broadcast address (host bits all 1) — only the two middle ones are assignable.
Four Key Addresses in a Subnet:

Address
Description
How to Find

Network Address
First address; identifies the network
AND the IP with subnet mask; all host bits = 0

First Usable Host
First assignable address
Network address + 1

Last Usable Host
Last assignable address
Broadcast address - 1

Broadcast Address
Last address; reaches all hosts
All host bits = 1

Example: 192.168.10.10/24

Address Type
Value

Network
192.168.10.0

First Usable
192.168.10.1

Last Usable
192.168.10.254

Broadcast
192.168.10.255

Usable hosts = 2^n - 2 (where n = number of host bits)

For /24: 2^8 - 2 = 254 usable hosts

Go deeper:

ipcalc (jodies.de) — enter any IP/mask and it prints the network, first/last host, and broadcast.
 Broadcast address (Wikipedia) — why the all-1s host pattern is the broadcast and is never assignable.

Address	Description	How to Find
Network Address	First address; identifies the network	AND the IP with subnet mask; all host bits = 0
First Usable Host	First assignable address	Network address + 1
Last Usable Host	Last assignable address	Broadcast address - 1
Broadcast Address	Last address; reaches all hosts	All host bits = 1

Address Type	Value
Network	192.168.10.0
First Usable	192.168.10.1
Last Usable	192.168.10.254
Broadcast	192.168.10.255