255.255.255.255 differs from a subnet's own broadcast address, such as 192.168.1.255 for 192.168.1.0/24: both reach every device on the local network, but 255.255.255.255 works even before a device knows which subnet it's on. That's exactly why DHCP relies on it. As a subnet mask, 255.255.255.255 means /32: a single host.
Facts
- Address
- 255.255.255.255
- Owner
- Reserved limited-broadcast address
On a Mac
Your Mac uses it during DHCP: before it has an address, it broadcasts a DHCP Discover to 255.255.255.255 to find the router. The default macOS /etc/hosts maps it to broadcasthost.
Frequently asked questions
What is 255.255.255.255 used for?
Broadcasting to every device on the local network, most importantly for DHCP, when a device without an address looks for a DHCP server.
Is 255.255.255.255 a valid IP?
Not for a device. It's a special broadcast destination; routers don't forward it beyond the local network.
What is a 255.255.255.255 subnet mask?
It's /32: a network of exactly one address, used for single-host routes and firewall rules.