/20 Subnet Mask — 255.255.240.0

The number after the slash counts the network bits, so a /20 has 20 network bits and 12 host bits. Those 12 bits are what produce the address count: 2^12 = 4,096 addresses in the block.

Prefix        /20
Subnet mask   255.255.240.0
Wildcard      0.0.15.255
Binary mask   11111111.11111111.11110000.00000000
Host bits     12

A common ISP or large-campus allocation, 4,096 addresses.

Taking 192.168.0.0 as the starting point and applying the /20 mask gives the boundaries below. The network address is the base with every host bit cleared, and the broadcast address is the base with every host bit set — which is why they are always the first and last addresses in the block.

To decide whether a /20 fits a requirement, compare the usable host count against the number of devices you need to address, then add spare capacity.

Splitting a /20 by borrowing one more bit produces two /21 blocks, each half the size. Borrowing two bits produces four /22 blocks, and so on. The counts are always powers of two.

Use the VLSM planner when the subnets inside a /20 need to be different sizes — for example a /25 for a user VLAN and several /30s for the links between switches.

• Forgetting the two reserved addresses. A /20 has 4,096 addresses but only 4,094 assignable to devices. Sizing a subnet at exactly 4,094 hosts of requirement leaves nothing for the network and broadcast address.
• Writing the mask wrong. There is exactly one valid mask for a /20: 255.255.240.0. Anything else is a different prefix and will silently create a different network.
• Using the wildcard as the mask. 0.0.15.255 is the wildcard for a /20, not the mask. In a Cisco ACL the wildcard convention is inverted, so the two get swapped constantly.
• Starting the subnet off its boundary. A /20 must start on a multiple of 4,096. Starting anywhere else overlaps neighbouring subnets and produces routing loops or unreachable ranges.
• Assuming a /20 is the right size for every case. Check the host requirement first — the CIDR calculator has a host-to-prefix cheat sheet, and the subnet calculator shows the full breakdown for any address.