/10 Subnet: Mask 255.192.0.0

A /10 splits the second octet into four blocks of 64: 0, 64, 128 and 192. The best-known /10 by far is 100.64.0.0/10, the shared address space ISPs use for carrier-grade NAT.

/10 at a glance

Subnet mask
255.192.0.0
Wildcard mask
0.63.255.255
Usable hosts
4,194,302
Total addresses
4,194,304 (222)
Block size (second octet)
64
Network / host bits
10 / 22
Mask in hex
0xFFC00000

Mask in binary: 11111111.11000000.00000000.00000000

Network bits Host bits

Try it: which /10 is an address in?

Network address
100.64.0.0/10
First usable host
100.64.0.1
Last usable host
100.127.255.254
Broadcast address
100.127.255.255
Next /10 starts at
100.128.0.0
Open this in the full subnet calculator (binary view, exports, cloud reserved IPs) →

How many /10 networks fit

How /10 relates to /8, /16 and /24 blocks
Parent block Relationship
/8 4 × /10 fit inside one /8
/16 One /10 covers 64 × /16
/24 One /10 covers 16,384 × /24

A /10 holds 64 × /16. Four /10s make a /8, which is why it is a neat size for splitting 10.0.0.0/8 into four regions.

Worked example: 100.101.7.42/10

A laptop shows the address 100.101.7.42. Is it in the CGNAT range 100.64.0.0/10, and where does that range end?

  1. The mask is 255.192.0.0. The second octet (192) is the "interesting" one — the first that is not 255.
  2. Block size = 256 − 192 = 64. Networks start at multiples of 64 in the second octet: 0, 64, 128, …
  3. The second octet of 100.101.7.42 is 101. 101 ÷ 64 = 1 remainder 37, so the block starts at 1 × 64 = 64.
  4. Network = 100.64.0.0 (octets after the second become 0). Broadcast = 64 + 64 − 1 = 127, giving 100.127.255.255 (octets after it become 255).
  5. Usable hosts: 100.64.0.1 to 100.127.255.254, which is 222 − 2 = 4,194,302 addresses.

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

Address  01100100.01|100101.00000111.00101010
Mask     11111111.11|000000.00000000.00000000
Network  01100100.01|000000.00000000.00000000

Where /10 is used in real networks

  • Carrier-grade NAT (RFC 6598)

    100.64.0.0/10 is reserved for ISPs to number customer routers that sit behind a large NAT. If your router's WAN address starts with 100.64 to 100.127, you are behind CGNAT and inbound port forwarding will not work. Details on the 100.64.0.0/10 page.

  • Tailscale and overlay networks

    Tailscale gives every device an address from 100.64.0.0/10 because it is unlikely to clash with home or office LANs.

  • Four regions out of one /8

    10.0.0.0/10, 10.64.0.0/10, 10.128.0.0/10 and 10.192.0.0/10 give you four equal regions (for example Americas, EMEA, APAC and cloud), each summarised by one route.

Common /10 mistakes

  • Using 100.64.0.0/10 as "spare private space" for your LAN. It is not RFC 1918; it clashes with ISP CGNAT and with Tailscale, and some software treats it as public.
  • Writing 100.100.0.0/10. The second octet must be 0, 64, 128 or 192. 100 is not a multiple of 64, so the block is really 100.64.0.0/10.
  • Thinking 100.64.0.0/10 ends at 100.64.255.255. It runs to 100.127.255.255 — 64 values of the second octet.

/10 FAQ

How many hosts does a /10 have?

4,194,302 usable addresses: 2^22 = 4,194,304, minus network and broadcast.

Is 100.64.0.0/10 private?

It is not RFC 1918 private space. It is "shared address space" from RFC 6598, meant for ISPs running carrier-grade NAT. It is not routed on the public Internet, but you should not use it for your own LAN.

Where does a /10 starting at 100.64.0.0 end?

At 100.127.255.255. The block size is 64 in the second octet, so it covers second-octet values 64 to 64 + 64 − 1 = 127.

Related tools

Subnet Calculator Full /10 breakdown with binary and exports Wildcard Mask Calculator ACL and OSPF wildcard for /10 and every other prefix CIDR Calculator Split 100.64.0.0/10 into smaller blocks VLSM Calculator Mix /10 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 /10 networks: 100.64.0.0/10