When a class C address such as 192.168.54.0 is subnetted with a /27 mask, the subnet mask in dotted decimal format is 255.255.255.224. This means that the interval between the network IDs of the resulting subnets is 32. The resulting network IDs are as follows:
192.168.54.0
192.168.54.32
192.168.54.64
192.168.54.92 and so on.

Therefore, the address 192.168.54.23 resides in the 192.168.54.0 subnet. The address 192.168.54.0 is called a network ID or, alternately, a subnet address. It represents the subnet as a group and will be used in the routing tables to represent and locate the subnet.

Neither the first address (192.168.54.0, the network ID) nor the last address (192.168.54.31, the broadcast address) in any resulting subnet can be used. Therefore, the addresses in this range are 192.168.54.1 through 192.168.54.30, which includes the 192.168.54.23 address.

192.168.54.8 would only be a network ID if the mask were /29, which would result in an interval of 8 between network IDs.
However, even if a /29 mask were used, the 192.168.54.23 address would not fall in its range. The address range for a /29 mask would be 192.168.54.9 through 192.168.54.14.

Similarly, 192.168.54.4 would only be a network ID for a /30 mask, which would result in an interval of 4 between network IDs. But even if a /30 mask were used, the 192.168.54.23 address would not fall in its range. The address range for a /30 mask would be 192.168.54.5 through 192.168.54.6.

192.168.54.16 could be a network ID if the mask were /28, /29 or /30, but not with a /27 mask.