0.0.0.0/0 · 0.0.0.0
Open in the calculator
0.0.0.0/0 means every IPv4 address. It appears as the default route in routing tables, as "anywhere" in firewall and security-group rules, and as the catch-all in allowlists. A security group that allows port 22 from 0.0.0.0/0 exposes SSH to the whole internet; restrict it to your own ranges.
On Linux, ip route show default prints the default route, which is the 0.0.0.0/0 entry pointing at your gateway. In cloud route tables the same entry sends internet-bound traffic to an internet gateway or NAT gateway. The IPv6 equivalent is ::/0, and a rule that is meant to cover "everything" needs both.
Key numbers
| Prefix length | /0 (0 network bits, 32 host bits) |
| Subnet mask | 0.0.0.0 |
| Mask in binary | 00000000.00000000.00000000.00000000 |
| Wildcard mask | 255.255.255.255 |
| Total addresses | 4,294,967,296 |
| Usable hosts | 4,294,967,294 (theoretical) |
| Contains | 16,777,216 × /24 networks |
Example ranges
Private (RFC 1918) blocks of this size. Click one to open it in the calculator.
| Block | First address | Last address | Usable hosts |
|---|---|---|---|
| 0.0.0.0/0 | 0.0.0.0 | 255.255.255.255 | 0.0.0.1 – 255.255.255.254 |
How the numbers are derived
An IPv4 address has 32 bits. A /0 uses 0 of them for the network, leaving 32 host bits, so the block has 232 = 4,294,967,296 addresses. The subnet mask is 0 ones followed by 32 zeros, written in dotted decimal as 0.0.0.0; the wildcard mask 255.255.255.255 is its inverse, used in Cisco ACLs and OSPF configuration.
FAQ
Is 0.0.0.0/0 the same as "any"?
Yes. In firewalls, security groups and route tables it matches every IPv4 address. The IPv6 equivalent is ::/0.
Why is 0.0.0.0/0 on port 22 a problem?
It lets any address on the internet attempt SSH logins. Restrict to known ranges or use a bastion, VPN or session manager.