/16 subnet: mask 255.255.0.0, 65,536 addresses

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Updated Oct 8, 2026

Examples

IP address / CIDR / netmask

IPv4 or IPv6, with or without a prefix — 10.0.0.0/8, 2001:db8::/48, or a dotted netmask like 192.168.1.0 255.255.255.0. A bare address means one host (/32 or /128).

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fig. 10 — subnet-calculator · networking /16 — 65534 usable hosts

Result

IPv410.0.0.0/16

Usable hosts65 534

Usable range10.0.0.1 – 10.0.255.254

Total addresses65 536

Addressing

Network address: 10.0.0.0
Broadcast address: 10.0.255.255
Usable host range: 10.0.0.1 – 10.0.255.254

Masks

Netmask: 255.255.0.0
Wildcard maskInverse of the netmask — the match form Cisco ACLs and OSPF expect.: 0.0.255.255
Netmask (binary)The mask bit by bit — the 1s are the network part.: 11111111.11111111.00000000.00000000

Details

Address type: Private (RFC 1918)
Network (integer)The network address as a single 32-bit number, as scripts and databases store it.: 167 772 160

Check against CIDRs Split this subnet Convert this address

A /16 fixes the first two octets and leaves the last two free: 65,536 addresses, 65,534 of them usable hosts. It is the classic size for a whole VPC or site, rarely for a single LAN.

What /16 means

The prefix length counts the leading 1 bits in the mask. Sixteen ones give 255.255.0.0, so the network part is the first two octets and the host part is the last two. The wildcard mask, the inverse form ACLs and OSPF use, is 0.0.255.255.

Host math is 2^(32 − 16) = 2^16 = 65,536 addresses. Subtract the network address and the broadcast address and 65,534 remain. For the example 10.0.0.0/16, usable hosts run from 10.0.0.1 to 10.0.255.254 and the broadcast is 10.0.255.255.

Where a /16 belongs

A /16 is an address plan, not a broadcast domain. Putting sixty-five thousand hosts on one flat segment would drown them in ARP and broadcast traffic long before the addresses ran out. Teams allocate a /16 to a VPC, a data centre or a region and carve it into /24s, /22s or /20s for the actual subnets.

On AWS a /16 is the largest CIDR block a VPC can have, and also the largest subnet. AWS reserves five addresses in every subnet (network, VPC router, DNS, one held for future use, broadcast), so a single /16 subnet would offer 65,531 assignable addresses. Azure reserves the same five.

Splitting and neighbours

One /16 holds 256 /24s, 16 /20s or 64 /22s. A common VPC layout reserves the low /20s for public subnets and higher ones for private and database tiers, leaving room to grow per availability zone.

Pick the /16 with peering in mind. Two VPCs that both use 10.0.0.0/16 can never be peered or joined over a VPN without NAT, so a registry of which /16 belongs to which environment saves a painful renumbering later.

Configuring a /16

On Linux an address with this mask goes on an interface as ip addr add 10.0.0.1/16 dev eth0; routers and firewalls accept either the prefix or the dotted mask. In practice a /16 rarely sits on an interface at all: it appears as a VPC CIDR, a summary route or an entry in an IPAM tool, and the interfaces live on smaller subnets cut from it.

A summary route for the /16 lets a remote site reach every subnet inside it with one line in its routing table, which is the main operational payoff of allocating space in /16 units.

FAQ

Questions, answered.

Tap a question to expand the answer.

How many /24 subnets fit in a /16?

256. Each /24 takes the third octet as its subnet number, from x.x.0.0/24 to x.x.255.0/24.

Is a /16 the same as an old Class B network?

It has the same size and mask (255.255.0.0), but CIDR lets a /16 start anywhere on a 16-bit boundary, not only in the old 128-191 first-octet range.

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