In the IPv6 address 2001:0db8:1234:0016:0023:8080:2345:88ab/64, what is the subnet quartet?

The first 48 bits of an IPv6 address are the global prefix; the next 16 bits are the subnet portion of the IPv6 address. 48 bits + 16 bits = 64 bits for the network ID. 1234 is the site ID, which is a portion of the network ID. 0023 is the first 16 bits of the interface ID. 8080 is the second 16 bits of the interface ID.