A /18 subnet uses 18 bits for the network portion of an IPv4 address, leaving 14 bits for host addressing. This produces 16,384 total IP addresses and 16,382 usable hosts. A /18 sits between the common /16 and /20, making it useful for large regional allocations and multi-site enterprise networks that need more than a /20 but don't require a full /16.
x.x.x.0 (third octet must be multiple of 64: .0, .64, .128, .192)
Broadcast address
x.x.63.255
Binary mask
11111111.11111111.11000000.00000000
AWS usable hosts
16,379 (AWS reserves 5 IPs per subnet)
Only 4 /18s fit in a /16
A /18 divides a /16 into exactly 4 equal blocks — third octets .0–.63, .64–.127, .128–.191, and .192–.255. This makes /18 a clean way to partition a /16 into four large zones: for example, splitting a VPC into four regions or business units, each getting their own /18 super-block to subdivide as they see fit.
The Four /18 Blocks in a 10.0.0.0/16
Subnet
Range
Third-Octet Span
Contains /24s
10.0.0.0/18
10.0.0.0 – 10.0.63.255
.0–.63
64
10.0.64.0/18
10.0.64.0 – 10.0.127.255
.64–.127
64
10.0.128.0/18
10.0.128.0 – 10.0.191.255
.128–.191
64
10.0.192.0/18
10.0.192.0 – 10.0.255.255
.192–.255
64
Common Use Cases for /18 Subnets
Multi-campus enterprise allocation — one /18 per site within a shared /16 address plan
Large cloud regions — a /18 per AWS region inside a corporate 10.x.x.x/8 space provides room for many VPCs
ISP customer blocks — medium-sized customers may receive a /18 allocation from their ISP
University networks — a /18 per faculty or major building cluster within a campus /16
Data centre super-net — aggregate 16 /22 blocks or 4 /20 blocks under one announcement