What a /10 subnet is: the carrier-grade NAT block
A /10 holds 4,194,304 addresses. Its one reserved use is 100.64.0.0/10, the shared address space set aside for carrier-grade network address translation.
At a glance
- Subnet mask
- 255.192.0.0
- Wildcard mask
- 0.63.255.255
- Total addresses
- 4,194,304
- Usable hosts
- 4,194,302
- Contains
- 16,384 networks of /24
- Host bits
- 22 of 32
In detail
The mask is 255.192.0.0, so the first octet and the top two bits of the second are fixed. RFC 6598 reserved this block so that internet providers could run large translation tiers without colliding with a customer's own private addressing.
If your public address is in 100.64.0.0/10, you are behind carrier-grade translation and you do not have a routable address of your own. Inbound connections and port forwarding will not work.
Do not use 100.64.0.0/10 for internal addressing just because it is spare. It is not private space, it belongs to the provider tier, and using it internally breaks the moment your provider puts you behind it.
Examples
100.64.0.0/10 shared address space, 100.64.0.0 to 100.127.255.255
# a WAN interface with a shared address means no inbound reachability
inet 100.83.14.7 netmask 255.192.0.0Things that catch people out
- 100.64.0.0/10 is often mistaken for private space. It is neither private nor publicly routable, and RFC 6598 is explicit that it must not be used on both sides of a translator.
- Some mesh and overlay networking products also use 100.64.0.0/10 for their virtual addresses, which collides with a provider that has put you behind translation.
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 /8 subnet is: a full class A block
A /8 uses the mask 255.0.0.0 and gives 16,777,214 usable addresses. What it is for and where it goes wrong.
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 /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.
What a /20 subnet is: sixteen /24s
A /20 uses the mask 255.255.240.0 and gives 4,094 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