The first usable address is the first IPv4 address in a network or subnet that can be assigned to a host. It is found by adding 1 to the network address. The network address itself identifies the network and cannot be assigned to a host. section 7.3.3 section 11.2.1
Use the surrounding address attributes to calculate and check it:
- Identify the network address, whose host portion contains all 0s. section 7.3.3
- Add 1 to the network address to obtain the first usable address. section 11.2.1
- Keep the broadcast address unavailable for hosts; the last usable address is one less than the broadcast address. section 11.2.1
Example: 192.168.1.64/26
The subnet has network address 192.168.1.64 and broadcast address 192.168.1.127. Therefore, its first usable address is 192.168.1.65, calculated as 192.168.1.64 + 1. Hosts can use addresses through 192.168.1.126.
