/21 Subnet Mask Reference (255.255.248.0)
Custom CIDR block with 2,048 total IPv4 addresses (2,046 usable host IPs) for VLSM allocation.
| CIDR Prefix | /21 |
|---|---|
| Subnet Mask | 255.255.248.0 |
| Wildcard Mask (Cisco / ACL) | 0.0.7.255 |
| Total IPv4 Addresses | 2,048 |
| Usable Host IPs | 2,046 |
| Binary Subnet Mask | 11111111.11111111.11111000.00000000 |
| Example Block (10.0.0.0/21) | 10.0.0.0 – 10.0.7.255 |
| Example Usable Hosts | 10.0.0.1 – 10.0.7.254 |
Need to compute custom IP ranges? Try the Interactive Subnet Calculator or VLSM Splitter.
Understanding the IPv4 /21 CIDR Block
An IPv4 /21 prefix allocates 21 network bits and 11 host bits out of a 32-bit IPv4 address, yielding an address block of 2^11 = 2,048 total IPv4 addresses. Expressed in dotted-decimal notation, the subnet mask is 255.255.248.0, and its bitwise inverse—used in Cisco IOS access control lists (ACLs) and OSPF area statements—is wildcard mask 0.0.7.255.
In standard Ethernet and cloud VPC routing, the first address in every /21 block identifies the subnet itself and the final address serves as the directed broadcast address, leaving 2,046 assignable host IPs on traditional routers (note that AWS VPC, Azure VNet, and GCP reserve 3 additional internal infrastructure IPs per subnet).