240.0.0.0/4 — the forever-reserved Class E
240.0.0.0/4 — 268,435,456 addresses from 240.0.0.0 to 255.255.255.255 — is "Class E", reserved by RFC 1112 in 1989 for future use. That future never came: not one address has ever been allocated, and the block sits frozen while the rest of IPv4 trades for real money.
Its very last address is famous, though: 255.255.255.255 is the limited broadcast address — "everyone on this local link" — hard-coded into DHCP and decades of software.
| Range start | 240.0.0.0 |
| Range end | 255.255.255.255 |
| Subnet mask | 240.0.0.0 |
| Wildcard mask | 15.255.255.255 |
| Total addresses | 268,435,456 |
Reserved Class E space with no assignments — shown as raw boundaries; only the final address, 255.255.255.255 (limited broadcast), has a live role.
The reclassification debate
Every few years someone proposes un-reserving 240/4 to relieve IPv4 exhaustion — 268 million addresses is a serious haul, sixteen legacy /8s' worth. The catch is that "Class E is invalid" is baked into a geological layer of TCP/IP stacks, routers and operating systems going back to the 1980s. Windows, for instance, refuses to configure it at all. Even if IANA opened the block, half the Internet could not talk to it — which makes the addresses close to worthless in practice.
255.255.255.255, the limited broadcast
One address in the block does real work: 255.255.255.255, the limited broadcast address standardized by RFC 919 and RFC 1122. A packet sent there is delivered to every host on the local segment and never forwarded by routers. DHCP depends on it — a client with no address yet broadcasts its Discover to 255.255.255.255 and every DHCP server on the link hears it.
Because that one address is load-bearing, any "just reclassify 240/4" proposal has to carve out the top slice or break DHCP-era assumptions. Combined with legacy stacks rejecting the whole /4, the block stays reserved.
CIDR Calculator
Run the math on 240.0.0.0/4 — or any other block
Frequently asked questions
Can I use 240.0.0.0/4 for my private network?
You should not. Many operating systems and routers treat Class E as invalid outright — Windows will not accept it as a static address — so connectivity would be unreliable at best. Use RFC 1918 space instead.
What is 255.255.255.255?
The limited broadcast address: it targets every host on the local link and is never routed. It is how a DHCP client with no IP address yet reaches the network's DHCP servers.
Why wasn't 240/4 released when IPv4 ran out?
Decades of stacks hard-code Class E as invalid, so the addresses could not be configured or routed across huge parts of the Internet. The cleanup cost exceeded the benefit compared with IPv6 adoption and the IPv4 transfer market.
How many addresses would 240.0.0.0/4 add?
268,435,456 (2^28) — about six percent of the IPv4 space, equivalent to sixteen /8 blocks.