/18 Subnet
16K IPs — quarter of a /16
A /18 offers 16,384 addresses. Often used to divide a VPC or campus network into four large segments.
Common Use Cases
- → VPC tier segmentation
- → Large-office building network
Example /18 Networks
| CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|
| 10.0.0.0/18 | 10.0.0.0 | 10.0.0.1 | 10.0.63.254 | 10.0.63.255 | 16,382 |
| 172.16.0.0/18 | 172.16.0.0 | 172.16.0.1 | 172.16.63.254 | 172.16.63.255 | 16,382 |
| 172.31.0.0/18 | 172.31.0.0 | 172.31.0.1 | 172.31.63.254 | 172.31.63.255 | 16,382 |
| 192.168.0.0/18 | 192.168.0.0 | 192.168.0.1 | 192.168.63.254 | 192.168.63.255 | 16,382 |
| 192.168.0.0/18 | 192.168.0.0 | 192.168.0.1 | 192.168.63.254 | 192.168.63.255 | 16,382 |
| 10.0.0.0/18 | 10.0.0.0 | 10.0.0.1 | 10.0.63.254 | 10.0.63.255 | 16,382 |
How /18 Compares
| Prefix | Total IPs | Usable Hosts | Subnet Mask | Typical Use |
|---|---|---|---|---|
| /15 | 131,072 | 131,070 | 255.254.0.0 | — |
| /16 | 65,536 | 65,534 | 255.255.0.0 | Cloud VPC CIDR, campus network |
| /17 | 32,768 | 32,766 | 255.255.128.0 | Large segment within a /16 |
| /18 ◀ | 16,384 | 16,382 | 255.255.192.0 | VPC segment |
| /19 | 8,192 | 8,190 | 255.255.224.0 | — |
| /20 | 4,096 | 4,094 | 255.255.240.0 | AWS default subnet, medium office VLAN |
| /21 | 2,048 | 2,046 | 255.255.248.0 | Enterprise building / large app tier |
Frequently Asked Questions
How many IPs are in a /18?
A /18 subnet contains 16,384 total IP addresses and 16,382 usable host addresses. The network address (first IP) and broadcast address (last IP) are reserved and cannot be assigned to hosts.
What is the subnet mask for /18?
The subnet mask for a /18 is 255.255.192.0.
In CIDR notation, the prefix length /18 means
the first 18 bits of the address are the network portion.
The wildcard (inverse) mask is 0.0.63.255.
How many /18 subnets fit in a /17?
Exactly 2 — a /17 can be split into two /18 subnets of equal size. Each /18 subnet has 16,382 usable hosts, compared to 32,766 in the parent /17.