What a /8 subnet is: a full class A block
A /8 fixes the first octet and leaves the other three free: 16,777,216 addresses. In private networking it is 10.0.0.0/8, the largest private range there is.
At a glance
- Subnet mask
- 255.0.0.0
- Wildcard mask
- 0.255.255.255
- Total addresses
- 16,777,216
- Usable hosts
- 16,777,214
- Contains
- 65,536 networks of /24
- Host bits
- 24 of 32
In detail
The mask is 255.0.0.0, so every address sharing the first octet is on the same network. This is the old class A, and it is why 10.x.x.x is so commonly used as a flat private space.
10.0.0.0/8 is the private block from RFC 1918. 127.0.0.0/8 is loopback, which is a whole /8 rather than the single address most people assume.
Do not run a /8 as one broadcast domain. Carve it into /16s per site or per environment, then into /24s or smaller per subnet, and leave room between them so a later expansion does not force renumbering.
Examples
10.0.0.0/8 private, 10.0.0.0 to 10.255.255.255
127.0.0.0/8 loopback, the whole range answers locally
$ ping 127.9.9.9
PING 127.9.9.9: 56 data bytes
64 bytes from 127.9.9.9: icmp_seq=0 ttl=64Things that catch people out
- Every address in 127.0.0.0/8 is loopback, not just 127.0.0.1. Services bound to 127.0.0.2 are reachable locally and are easy to miss when auditing what is listening.
- Two organisations that both used 10.0.0.0/8 flat cannot be merged or connected by a VPN without renumbering or network address translation. This is the single most common cause of painful acquisitions in networking.
Try it: IP Subnet Calculator
Enter any IPv4 address with a prefix or a dotted mask and get the network address, broadcast address, wildcard mask, usable host range, address class, and the hex and binary forms.
IP Subnet Calculator
LOCAL · Browser-OnlyCalculate IP subnet details, network ranges, and CIDR notation
192.168.1.0192.168.1.255255.255.255.00.0.0.255192.168.1.0/24192.168.1.1 - 192.168.1.254192.168.1.1192.168.1.254256254C0xC0A8016411000000.10101000.00000001.01100100Split into two /25 subnets
Dividing 192.168.1.0/24 one prefix deeper.
Common Subnet Masks
Related IPv4 subnets
All ipv4 subnets →What a /12 subnet is: the 172.16 private block
A /12 uses the mask 255.240.0.0 and gives 1,048,574 usable addresses. What it is for and where it goes wrong.
What a /16 subnet is: a full class B block
A /16 uses the mask 255.255.0.0 and gives 65,534 usable addresses. What it is for and where it goes wrong.
What a /24 subnet is: the classic LAN subnet
A /24 uses the mask 255.255.255.0 and gives 254 usable addresses. What it is for and where it goes wrong.
What a /10 subnet is: the carrier-grade NAT block
A /10 uses the mask 255.192.0.0 and gives 4,194,302 usable addresses. What it is for and where it goes wrong.
What a /14 subnet is: a four-way /16 aggregate
A /14 uses the mask 255.252.0.0 and gives 262,142 usable addresses. What it is for and where it goes wrong.
What a /18 subnet is: a quarter of a /16
A /18 uses the mask 255.255.192.0 and gives 16,382 usable addresses. What it is for and where it goes wrong.
Tools for this
- IP Subnet Calculator Free online IP subnet calculator - calculate IP subnet details, CIDR notation, and network address ranges
- IP Address Lookup Free online IP address lookup - find your public IP address and basic network information
- Number Base Converter Free online number base converter - convert numbers between decimal, binary, octal, and hexadecimal