The subnet mask 255.255.254.0 (/23) means 23 bits for network, 9 bits for hosts. IP 172.21.23.7 has third octet 23 (00010111). Mask's third octet is 254 (11111110). AND operation gives 22 (00010110). Network address is 172.21.22.0. The /23 subnet spans 172.21.22.0 to 172.21.23.255.