What does 0.0.0.0/0 mean?

Answer

0.0.0.0/0 is a prefix of length zero, so no bits have to match: it covers every IPv4 address, all 4,294,967,296 of them. In a routing table it is the default route; in a firewall or security group rule it means "from anywhere".

0.0.0.0/0 is 4,294,967,296 addresses: 0.0.0.0 to 255.255.255.255. 4,294,967,294 usable for hosts (the first is the network address, the last is broadcast).

000000000000000000000000000000000000

0 network bits 32 host bits

Network: 0.0.0.0
First usable: 0.0.0.1
Last usable: 255.255.255.254
Broadcast: 255.255.255.255
Addresses: 4,294,967,296
Usable hosts: 4,294,967,294
Netmask: 0.0.0.0
Wildcard (ACL) mask: 255.255.255.255
Historic class: A
Range type: The entire IPv4 address space (the default route; 'anywhere' in firewall rules)

This block contains special-purpose ranges: 0.0.0.0/8 (This network), 10.0.0.0/8 (Private network), 100.64.0.0/10 (Shared address space (carrier-grade NAT)), 127.0.0.0/8 (Loopback), 169.254.0.0/16 (Link-local), 172.16.0.0/12 (Private network), 192.0.0.0/24 (IETF protocol assignments), 192.0.2.0/24 (Documentation (TEST-NET-1)) and more.

Split it into subnets

4 subnets.

Subnet	Usable range	Hosts	Broadcast
`0.0.0.0/2`	`0.0.0.1` – `63.255.255.254`	1,073,741,822	`63.255.255.255`
`64.0.0.0/2`	`64.0.0.1` – `127.255.255.254`	1,073,741,822	`127.255.255.255`
`128.0.0.0/2`	`128.0.0.1` – `191.255.255.254`	1,073,741,822	`191.255.255.255`
`192.0.0.0/2`	`192.0.0.1` – `255.255.255.254`	1,073,741,822	`255.255.255.255`

Is this IP in the range?

Range to CIDR blocks

What to know

Routers pick the longest matching prefix, and /0 is the shortest possible, so the default route is used only when nothing more specific matches. ip route add default via 192.168.1.1 on Linux installs exactly this route, and a cloud route table sends 0.0.0.0/0 to an internet or NAT gateway.

In an inbound rule, 0.0.0.0/0 exposes the port to the whole IPv4 internet. That is normal for ports 80 and 443 on a public web server and a common mistake for SSH (22), RDP (3389) and databases.

Gotchas

0.0.0.0/0 covers only IPv4. To match all IPv6 traffic too, you need a separate ::/0 rule, and if you only restrict IPv4, a service may still be reachable over IPv6.
The address 0.0.0.0 (no prefix) as a listen address means "all local interfaces". That is a different idea from the 0.0.0.0/0 network.
Host counts and a broadcast address computed for a /0 have no practical meaning: it is a matching rule, not a network anyone configures on an interface.

Related explainers

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