What is a /20 subnet? 4,096 IPs and the AWS default size

How this is calculated

In binary, the mask leaves 12 host bits, so the block holds 4,096 addresses. AWS's default VPC is a /16, and AWS says that VPC provides up to 65,536 private IPv4 addresses. A /16 holds 16 /20 blocks, and each /20 holds 16 /24 blocks. If a second building needs its own /20, the same /16 has room for it. Access control lists and some routing configs write this block with its wildcard mask, 0.0.15.255, which is the mask with every bit flipped. AWS and Azure each reserve five addresses in every subnet, so a /20 leaves 4,091 addresses for resources. With 10.0.16.0 as the network address, the usable range is 10.0.16.1 to 10.0.31.254, and the broadcast address is 10.0.31.255.

Verdict

Pick a /20 for a single campus building, a large floor or a segment you expect to split with VLANs later. Its 4,096 addresses leave room for many devices, but in a cloud subnet, plan around 4,091. A /20 also sits neatly inside a /16, so the rest of that block stays free for other subnets. If each VLAN takes a /24, this /20 is the summary the core router advertises for the whole building, and it covers 16 of them. Sketch the VLAN list before you carve the block, so each VLAN gets a size that matches its job.