An administrator is given the following network information to configure a server with a static IP address: 10.152.0.4/21. Which of the following is the subnet mask for the given IP address?

255.248.0.0 corresponds to a /13 prefix (13 ones), not /21, as only the first octet is fully masked and the second has 5 bits set.

255.255.128.0 corresponds to a /17 prefix (17 ones), where only the high bit of the third octet is set (10000000 = 128), not 5 bits.

A /21 CIDR prefix means the first 21 bits of the subnet mask are set to 1. The first two octets are fully masked (255.255), the third octet has the top 5 bits set (11111000 = 248), and the fourth octet is 0, yielding 255.255.248.0. This gives 2048 total addresses per subnet.

255.255.255.248 corresponds to a /29 prefix (29 ones), which masks into the fourth octet with 5 bits set, yielding only 8 addresses per subnet.