/8 Subnet Mask ReferenceIPv4
One fixed byte, three free: a /8 holds 16,777,216 addresses and echoes the Class A era when a single organization owned an entire first octet. Whole public /8s are scarce now, but two of them touch every engineer daily: <code>10.0.0.0/8</code> for private networks and <code>127.0.0.0/8</code> for loopback.
/8 facts at a glance
- Subnet mask
- 255.0.0.0
- Wildcard mask
- 0.255.255.255
- Binary mask
- 11111111.00000000.00000000.00000000
- Total addresses
- 16,777,216
- Usable hosts
- 16,777,214 (excludes network & broadcast)
- Block size
- 16,777,216 addresses (224)
- Typical role
- Legacy Class A equivalent
Example /8 networks
A /8 spans a large share of the address space, so one aligned example says it all.
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.0.0.0/8 | 198.0.0.1 – 198.255.255.254 | 16,777,214 |
How to calculate a /8 mask
A /8 mask sets the first 8 bits of a 32-bit number to 1 and leaves the remaining 24 bits as 0. In binary that reads 11111111.00000000.00000000.00000000, which converts to dotted decimal 255.0.0.0. Each block covers a contiguous run of 224 = 16,777,216 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.255.255.255.
Where /8 is used
- Private enterprise addressing: RFC 1918's <code>10.0.0.0/8</code> is the default canvas for large internal networks.
- Loopback testing: any 127.x.x.x address loops back to the local machine (RFC 1122).
- Summarization roots for hyperscale operators whose internal maps are drawn in /8 strokes.
- Legacy public allocations from the classful period, still visible in whois records.
RFC 1918 reserves 10.0.0.0/8 for private internets: freely reusable behind NAT, but such packets must never surface on the public Internet.
History lesson: classful addressing handed out first octets 1 through 126 as indivisible /8s, leaving barely more than a hundred public blocks for the whole world. That shortage is precisely what CIDR was invented to fix.
One /8 subdivides into 256 /16s or 65,536 /24s, which keeps it useful as a top-down planning unit even when only fragments get allocated.
Frequently asked questions
How many hosts are in a /8?
16,777,214 usable of 16,777,216 total addresses. Two are withheld: the lowest names the network and the highest serves as the broadcast address.
What is the subnet mask for a /8?
<code>255.0.0.0</code>, with wildcard <code>0.255.255.255</code>. Only the first octet identifies the network; the rest belongs to hosts.
Why is 10.0.0.0/8 so popular internally?
It is the largest RFC 1918 pool, over 16.7 million private addresses, easy to remember and easy to subnet hierarchically, and guaranteed never to collide with public routing.
Is 127.0.0.1 part of a /8?
Yes. The entire 127.0.0.0/8 block is loopback under RFC 1122; 127.0.0.1 is merely its best-known member, and every other 127.x.x.x address behaves identically.
Open this block in a calculator
Open the /8 block in the subnet calculator