How many /22 networks fit
| Parent block | Relationship |
|---|---|
| /8 | 16,384 × /22 fit inside one /8 |
| /16 | 64 × /22 fit inside one /16 |
| /24 | One /22 covers 4 × /24 |
A /22 holds 4 × /24. Four /22s make a /20.
Worked example: 192.168.6.31/22
An office moved from 192.168.4.0/24 to a /22 to fit more devices. A laptop at 192.168.6.31 can't reach the printer. Is it even in the same /22?
- The mask is 255.255.252.0. The third octet (252) is the "interesting" one — the first that is not 255.
- Block size = 256 − 252 = 4. Networks start at multiples of 4 in the third octet: 0, 4, 8, …
- The third octet of 192.168.6.31 is 6. 6 ÷ 4 = 1 remainder 2, so the block starts at 1 × 4 = 4.
- Network = 192.168.4.0 (octets after the third become 0). Broadcast = 4 + 4 − 1 = 7, giving 192.168.7.255 (octets after it become 255).
- Usable hosts: 192.168.4.1 to 192.168.7.254, which is 210 − 2 = 1,022 addresses.
Same thing in binary (network bits | host bits):
Address 11000000.10101000.000001|10.00011111 Mask 11111111.11111111.111111|00.00000000 Network 11000000.10101000.000001|00.00000000
Where /22 is used in real networks
-
Growing past a /24
When a VLAN runs out of 254 addresses, widening it to a /22 is the usual fix, as long as the four /24s are free and aligned (192.168.4.0 to 192.168.7.255, not 192.168.3.0 to 192.168.6.255).
-
Public IPv4 allocations
When RIPE NCC ran its "last /8" policy, each new LIR received a single /22 of IPv4. Many small ISPs and hosting companies still run on exactly one /22.
Common /22 mistakes
- Writing
192.168.1.0/22. The third octet of a /22 must be a multiple of 4, so it is really 192.168.0.0/22 — which includes your old 192.168.0.x and 192.168.1.x networks. - Changing the mask on the router but not on statically addressed devices. A printer still on /24 thinks 192.168.6.31 is on another network and sends replies to the gateway.
- Thinking 192.168.5.0 or 192.168.6.255 can't be used. Inside a /22 only 192.168.4.0 and 192.168.7.255 are reserved; the .0 and .255 in the middle are normal hosts.
/22 FAQ
How many hosts are in a /22?
1,022 usable: 2^10 = 1,024 addresses minus network and broadcast.
How many /24s are in a /22?
Four. A /22 has 2 more host bits than a /24, and 2^2 = 4, so a /22 such as 192.168.4.0/22 covers 192.168.4.x, .5.x, .6.x and .7.x.
What is the subnet mask for /22?
255.255.252.0. The first 22 bits are ones, which fills two octets with 255 and leaves 11111100 (252) in the third. The wildcard is 0.0.3.255.