← 1-3 Subnetting

Subnet Calculator

Type an address. Drag the line between network and host.

IP Address
. . .
Example Address
Prefix Length /24
/0/8/16/24/32
192
168
1
100
Network bits
Host bits
Network Address
192.168.1.0
Broadcast Address
192.168.1.255
First Usable Host
192.168.1.1
Last Usable Host
192.168.1.254
Usable Hosts
254
Mask and bits
CIDR Notation
192.168.1.100/24
Subnet Mask
255.255.255.0
Network Bits
24 bits
Host Bits
8 bits

IPv6 Address
Example Address
Prefix Length /64
/0/16/32/48/64/80/96/112/128

IPv6 is read as eight 16-bit hextets. The prefix boundary can split a hextet at an exact bit.

Scroll to inspect all eight hextets
Routing prefixAggregates routes toward a site.
Subnet IDSelects a link inside the site.
Interface IDIdentifies an interface on a typical /64 link.

Canonical text uses lowercase, removes leading zeros, and compresses one longest zero run.

Expanded
Trimmed
Canonical
Rule 1Suppress leading zeros in each hextet.
Rule 2Use :: once on the longest run of two or more zero hextets.

Routing accepts any prefix length. /64 is the normal SLAAC subnet boundary.

Type and scope come from defined leading bit patterns.

TYPE
Scope
Defined block
Network prefix
Bits after the prefix
Full Address
Compressed
Network Prefix
First Address
Last Address
Addresses in Subnet
Prefix Bits
64 bits
Interface ID Bits
64 bits
Address Type
Global Unicast
How to read IPv6

How to Read an IPv6 Address

An IPv6 address is 128 bits written as 8 groups of 4 hex digits, separated by colons. Each group is called a hextet (16 bits).

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Two shorthand rules make them shorter:

  • Drop leading zeros in each hextet: 0db8 becomes db8, 0370 becomes 370
  • Replace one run of all-zero hextets with :: (only once per address): :0000:0000: becomes ::

So the address above compresses to: 2001:db8:85a3::8a2e:370:7334

Prefix vs Interface ID

In IPv6, there is no subnet mask in dotted-decimal form. Instead, you just use CIDR prefix length (the /64 part).

  • Prefix (network portion): identifies which network the device belongs to. Drag the slider to change it.
  • Interface ID (host portion): identifies the specific device on that network. In a /64, this is the last 64 bits.

Unlike IPv4, there is no broadcast address in IPv6. Instead, IPv6 uses multicast (ff02::1 for all nodes). Every address in the range is usable.

Common IPv6 Prefix Lengths

  • /128 - a single host (loopback ::1, or a specific device route)
  • /64 - the standard subnet. SLAAC (auto-config) requires this. Almost every LAN uses /64.
  • /48 - typical allocation from an ISP to a customer site. Gives you 65,536 /64 subnets.
  • /32 - allocation from a RIR to an ISP. 4 billion subnets available to hand out.
  • /10 - link-local scope (fe80::/10). Auto-assigned on every interface, never routed.
  • /7 - unique local addresses (fc00::/7, practically fd00::/8). The IPv6 equivalent of RFC 1918 private space.

Key Differences from IPv4

  • No broadcast - IPv6 uses multicast and anycast instead. No more broadcast storms.
  • End-to-end addressing - IPv6 removes address scarcity as the reason for NAT. Firewalls and routing policy still control reachability.
  • Auto-configuration - SLAAC combines an advertised prefix with an interface identifier. Modern systems commonly use stable opaque or temporary identifiers instead of exposing a MAC address.
  • Simpler headers - fixed 40-byte header, no checksum (handled by link layer), no fragmentation by routers.
  • No IPv4-style -2 rule - IPv6 has no broadcast address. Special and anycast uses still matter, including the all-zero Subnet-Router anycast identifier on router subnets.

Practice Mode

Test what you know. The bits don't lie.

Question

10.4.12.200/22

What is the network address?