Given a subnet address of 10.6.128.0/22, how many IP addresses are usable for this subnet?

1024 is the total address count for a /22, not the usable count, because it ignores the subtraction of the network and broadcast addresses.

2048 is the total address count for a /21 subnet (2^11), which has 11 host bits, not 10.

A /22 prefix leaves 10 host bits (32-22=10), yielding 2^10 = 1024 total addresses. Two addresses are always reserved - the network address (10.6.128.0) and the broadcast address (10.6.131.255) - leaving exactly 1022 usable host addresses.

2046 is the usable host count for a /21 subnet (2048-2), not a /22.

256 addresses corresponds to a /24 subnet (2^8 = 256), not a /22.