How many usable IP addresses are there in a 20-bit subnet?

To determine the number of usable IP addresses in a 20-bit subnet, we first need to understand how subnetting works. A subnet mask of 20 bits means that 20 bits are used for the network portion of the IP address, leaving the remaining bits for host addresses.

An IPv4 address consists of 32 bits in total. With a 20-bit subnet mask, there are 12 bits remaining for hosts (32 - 20 = 12). The total number of possible combinations for these 12 bits is calculated as 2 raised to the power of 12, which equals 4096. This figure represents all possible addresses within the subnet, including network and broadcast addresses.

However, in any subnet, two addresses are reserved and cannot be used for hosts: the first address serves as the network identifier, and the last address is reserved as the broadcast address. Therefore, to determine the number of usable IP addresses, we subtract these two reserved addresses from the total.

Thus, the calculation becomes:

4096 total addresses - 2 reserved addresses = 4094 usable IP addresses.

This is why the correct answer is 4094, as it takes into account the necessary reservations in a 20-bit subnet.