The last usable address is the final IPv4 address that can be assigned to a host in a network or subnet. It is immediately before the broadcast address; the broadcast address itself cannot be assigned to a host. section 7.3.3 section 11.2.1
Find the last usable address with this practical sequence:
- Identify the subnet’s broadcast address, whose host portion contains all 1s. section 7.3.3
- Subtract 1 from the broadcast address; this produces the last address available for host assignment. section 11.2.1
- Remember that the network address and broadcast address are excluded, so the maximum host count is 2ᵞ − 2. section 7.3.3
Example: 192.168.1.64/26
The subnet’s broadcast address is 192.168.1.127. Subtracting 1 gives 192.168.1.126, the last usable address. The first usable address is 192.168.1.65, and the subnet supports 62 hosts. section 11.2.1
Figure 7.15 192.168.100.254 is the last usable address of the 192.168.100.0/24 network. It is the last address before the broadcast address.
Use the figure to compare the last usable address with the broadcast address in a /24 network. section 7.3.3
