/8 Subnet: Mask 255.0.0.0

A /8 fixes only the first octet. Everything after it — 24 bits, about 16.7 million addresses — belongs to one block. You will meet /8s as whole private ranges and old Class A allocations, almost never as a single network that devices sit on.

/8 at a glance

Subnet mask
255.0.0.0
Wildcard mask
0.255.255.255
Usable hosts
16,777,214
Total addresses
16,777,216 (224)
Block size (first octet)
1
Network / host bits
8 / 24
Mask in hex
0xFF000000

Mask in binary: 11111111.00000000.00000000.00000000

Network bits Host bits

Try it: which /8 is an address in?

Network address
10.0.0.0/8
First usable host
10.0.0.1
Last usable host
10.255.255.254
Broadcast address
10.255.255.255
Next /8 starts at
11.0.0.0
Open this in the full subnet calculator (binary view, exports, cloud reserved IPs) →

How many /8 networks fit

How /8 relates to /8, /16 and /24 blocks
Parent block Relationship
/8 Same size: a /8 is exactly one /8
/16 One /8 covers 256 × /16
/24 One /8 covers 65,536 × /24

In practice nobody puts 16 million devices in one broadcast domain. A /8 is an address plan, not a subnet: you split it into /16s per site or region and /24s or smaller per VLAN, and you advertise the /8 only as a summary route.

Worked example: 10.47.200.13/8

A firewall log shows traffic from 10.47.200.13 and a rule matches 10.0.0.0/8. Is the address inside it, and what are the block's edges?

  1. /8 is 1 full octet of network bits (mask 255.0.0.0), so the boundary falls exactly between octets. No block-size maths is needed.
  2. Keep the first 1 octet of 10.47.200.13 and set the rest to 0: network = 10.0.0.0.
  3. Set the rest to 255 instead: broadcast = 10.255.255.255.
  4. Usable hosts run from 10.0.0.1 to 10.255.255.254: 224 − 2 = 16,777,214.

Same thing in binary (network bits | host bits):

Address  00001010|00101111.11001000.00001101
Mask     11111111|00000000.00000000.00000000
Network  00001010|00000000.00000000.00000000

Where /8 is used in real networks

  • 10.0.0.0/8 — the largest private range

    RFC 1918 reserves all of 10.0.0.0/8 for private use. Large companies often use it as their whole internal address plan, handing a /16 to each site. See the 10.0.0.0/8 page for how to split it.

  • Loopback: 127.0.0.0/8

    Every address from 127.0.0.1 to 127.255.255.254 loops back to the local machine, not just 127.0.0.1. Ubuntu uses 127.0.1.1 for the hostname and systemd-resolved listens on 127.0.0.53.

  • Summary routes

    A core router or VPN head-end can carry one route for 10.0.0.0/8 that points at the internal network, instead of hundreds of more specific routes. Longest-prefix match still sends traffic to more specific routes when they exist.

Common /8 mistakes

  • Typing 10.0.0.0/8 in a security group or ACL when you meant 10.0.0.0/24. One wrong digit opens access to 16 million addresses instead of 256.
  • Assuming 10.0.0.0/8 is "yours". Partners, cloud VPCs and VPN users all use it too. Using the whole /8 everywhere guarantees overlap the day you connect to someone else.
  • Treating a /8 summary route as proof a subnet exists. A packet to 10.200.1.1 will follow the /8 route even if nothing is using 10.200.0.0, and it will be dropped somewhere deep in the network instead of failing fast.

/8 FAQ

How many usable hosts are in a /8?

16,777,214. A /8 has 24 host bits, so 2^24 = 16,777,216 addresses, minus the network address and the broadcast address.

Is /8 the same as Class A?

The mask is the same (255.0.0.0), but classes were retired in 1993 by CIDR. Today /8 is just a prefix length; any network can be a /8 if it starts on a /8 boundary.

How many /24 networks are in a /8?

65,536. Each extra bit doubles the count, and going from /8 to /24 adds 16 bits: 2^16 = 65,536.

Can I use 10.0.0.0/8 as one flat network?

Technically yes, but you should not. Broadcast and ARP traffic would reach every device, and one misbehaving host could flood the whole network. Split it into smaller subnets and route between them.

Related tools

Subnet Calculator Full /8 breakdown with binary and exports Wildcard Mask Calculator ACL and OSPF wildcard for /8 and every other prefix CIDR Calculator Split 10.0.0.0/8 into smaller blocks VLSM Calculator Mix /8 with other sizes in one plan Subnet Mask Calculator Convert mask ↔ prefix ↔ host count Subnetting Practice Drill questions like this one, with visual answers

Well-known /8 networks: 10.0.0.0/8, 127.0.0.0/8