Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
A /21 subnet contains 2,046 valid host addresses. A subnet mask specifies how many bits belong to the network portion of a 32bit IP address. The remaining bits in the IP address belong to the host portion of the IP address. To determine how many host addresses are defined by a subnet mask, use the formula 2n-2, where n is the number of bits in the host portion of the address. You must subtract 2 from the number of available hosts, because the first address is the subnetwork address and the last address is the broadcast address.
To determine the number of bits in the host portion of the address, you should convert /21 to dotted- decimal notation. To convert /21 from Classless Inter-Domain Routing (CIDR) notation to dotted-decimal notation, begin at the left and set the first 21 bits to a value of 1. These bits identify the network portion of the IP address. The remaining 11 bits will be set to 0. These are the host bits.
/21 = 11111111.11111111.11111000.00000000
There are 11 bits equal to the /21 subnet mask. Applying the 2n-2 formula, where n = 11, yields 2,048 -2
2,046. Therefore, 2,046 hosts are available for each subnetwork when a subnet mask of /21 is applied.
Although it is important to learn the formula for calculating valid host addresses, the following list demonstrates the relationship between common subnet masks and valid host addresses:

Subnetting a contiguous address range in structured, hierarchical fashion enables routers to maintain smaller routing tables and eases administrative burden when troubleshooting. Conversely, a discontiguous IP version 4 (IPv4) addressing scheme can cause routing tables to bloat because the subnets cannot be summarized. Summarization minimizes the size of routing tables and advertisements and reduces a router's processor and memory requirements.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 8, Plan for a Hierarchical IP Address Network, pp. 311-312 Cisco: IP Addressing and Subnetting for New Users