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 Explanation & Hint: To understand why the network ID is 10.128.0.0 and the subnet mask is 255.128.0.0 for the IP address 10.200.156.22/9, let’s break down the calculation process: CIDR Notation (/9): The “/9” after the IP address indicates that the first 9 bits of the IP address are designated for the network portion. The remaining bits (32 – 9 = 23 bits) are used for host addresses within that network. Subnet Mask Calculation: A subnet mask identifies the network portion of an IP address by having binary 1s in all positions that correspond to the network bits, followed by binary 0s for the host bits. For a /9 prefix, the subnet mask has the first 9 bits set to 1, and the remaining 23 bits set to 0. In binary, this subnet mask looks like `11111111 10000000 00000000 00000000`, which translates to 255.128.0.0 in decimal. This indicates that the first 9 bits are used for network identification and the rest for host identification within that network. Network ID Calculation: The network ID is calculated by taking the IP address and applying the subnet mask, essentially retaining the network portion of the IP address and setting all host bits to 0. For the IP address 10.200.156.22, when we convert it into binary and then keep only the first 9 bits while setting the remaining bits to 0, we align with the subnet mask’s indication of the network portion. The result in binary for the network portion up to the 9th bit corresponds to `00001010 10000000 00000000 00000000` in binary, which translates to 10.128.0.0 in decimal. This process shows that the network ID is 10.128.0.0, which means that any IP address within the range defined by the first 9 bits (in this case, starting from 10.128.0.0 to a certain point determined by the remaining bits) belongs to the same network. The subnet mask of 255.128.0.0 is used to delineate the boundary of this network, indicating which part of an IP address represents the network and which part can be assigned to hosts within that network.