What does /16 mean?
A /16 keeps the first 16 bits (the first two numbers) fixed and leaves 16 bits for hosts: 65,536 addresses, 65,534 usable as hosts. The netmask is 255.255.0.0.
Worked example: 172.31.0.0/16. Type any block to compare.
172.31.0.0/16 is 65,536 addresses: 172.31.0.0 to 172.31.255.255. 65,534 usable for hosts (the first is the network address, the last is broadcast).
- Network
172.31.0.0- First usable
172.31.0.1- Last usable
172.31.255.254- Broadcast
172.31.255.255- Addresses
65,536- Usable hosts
65,534- Netmask
255.255.0.0- Wildcard (ACL) mask
0.0.255.255- Historic class
B- Range type
Private network · not routed on the internet
Special-purpose range
Private network 172.16.0.0/12 RFC 1918
The middle private IPv4 range: 172.16.0.0 to 172.31.255.255. Docker's default bridge networks are carved from it.
Split it into subnets
4 subnets.
| Subnet | Usable range | Hosts | Broadcast |
|---|---|---|---|
172.31.0.0/18 | 172.31.0.1 – 172.31.63.254 | 16,382 | 172.31.63.255 |
172.31.64.0/18 | 172.31.64.1 – 172.31.127.254 | 16,382 | 172.31.127.255 |
172.31.128.0/18 | 172.31.128.1 – 172.31.191.254 | 16,382 | 172.31.191.255 |
172.31.192.0/18 | 172.31.192.1 – 172.31.255.254 | 16,382 | 172.31.255.255 |
Is this IP in the range?
Range to CIDR blocks
What to know
/16 is the old Class B size and the most common size for a whole private network plan per environment, region or site. 192.168.0.0/16 is one of the three private ranges.
On AWS, /16 is the largest CIDR block a VPC can have (a VPC can be anything from /16 to /28). The default VPC in every region is 172.31.0.0/16, split into one /20 subnet per availability zone.
Docker's default bridge, docker0, is 172.17.0.0/16, and new Docker networks take further /16s from the 172.16.0.0/12 range by default.
Gotchas
- A /16 VPC cannot be extended in place; AWS lets you add secondary CIDR blocks, but plan the size before peering, since overlapping VPCs cannot be peered.
- Using the same 10.0.0.0/16 in every VPC or office is the most common cause of VPN and peering conflicts later. Give each one its own range.
Related explainers
- What does /8 mean?A /8 fixes the first octet and leaves 24 bits for hosts: 16,777,216 addresses, netmask 255.0.0.0. The old...
- What does /20 mean?A /20 has 4,096 addresses (4,094 usable), netmask 255.255.240.0: sixteen /24s. Why AWS default subnets are...
- What does /24 mean?A /24 has 256 addresses and 254 usable hosts with netmask 255.255.255.0. The typical home and office LAN,...
- Private IPv4 address ranges (RFC 1918)The three private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Their sizes, exact...