Visual lab

Learn subnetting visually

11 short lessons, from binary to IPv6. Change the inputs and watch every value update.

Lesson 11 of 11Intermediate

IPv6 basics

Why IPv6?

IPv4 has about 4.3 billion addresses, and they ran out. IPv6 addresses are 128 bits, written as eight groups of four hex digits called hextets, like 2001:db8::1.

Expand and compress

Type any IPv6 address, short or long. The two shortening rules are applied one at a time.

IPv6 address

8 hextets of 16 bits each

2001

bits 1–16

0db8

bits 17–32

0000

bits 33–48

0000

bits 49–64

0008

bits 65–80

0800

bits 81–96

200c

bits 97–112

417a

bits 113–128

Full form
2001:0db8:0000:0000:0008:0800:200c:417a
Rule 1: drop leading zeros in each hextet
2001:db8:0:0:8:800:200c:417a
Rule 2: replace the longest run of zero hextets with :: (hextets 3–4)
2001:db8::8:800:200c:417a

:: may appear only once, or nobody could tell how many zeros each one hides. When two zero runs tie, the first one is compressed. A single zero hextet is written as 0, not ::. Lowercase is the standard form (RFC 5952).

Subnetting IPv6

IPv6 subnetting is about counting subnets, not hosts. A LAN is a /64: the last 64 bits identify the interface.

Prefix	Typical use	LANs inside	Why
/48	Typical site allocation	65,536 /64s	64 − 48 = 16 bits of subnet ID
/52	Nibble boundary	4,096 /64s	12 bits
/56	Common for small sites and homes	256 /64s	8 bits
/60	Nibble boundary	16 /64s	4 bits
/64	One LAN segment	1	Host part is the last 64 bits

Example: site 2001:db8:abcd::/48, one hex digit = 4 bits

2001:db8:abcd:0000::/64

The fourth hextet is the 16-bit subnet ID. Each prefix on a multiple of 4 (a nibble boundary: /52, /56, /60) ends exactly at one hex digit, so subnets read cleanly: 2001:db8:abcd:1200::/56 holds 2001:db8:abcd:1200::/64 to 2001:db8:abcd:12ff::/64.

2001:db8::/32 is reserved for documentation (RFC 3849), which is why examples use it.

No broadcast in IPv6

IPv6 has no broadcast address. Messages for many hosts use multicast instead, and Neighbor Discovery (which replaces ARP) sends to multicast groups so only the interested hosts process them.

Address types to recognize

The first few hex digits tell you what kind of address it is.

2000::/3Global unicast

Internet-routable addresses. Today's global allocations start with 2 or 3.

fe80::/10Link-local

Every IPv6 interface has one. Valid only on its own link; routers never forward it.

fc00::/7Unique local

Private addressing for internal use, in practice fd00::/8. Not routed on the internet.

ff00::/8Multicast

ff02::1 reaches all nodes on the link, ff02::2 all routers.

::1/128Loopback

The IPv6 version of 127.0.0.1.

::/128Unspecified

Means no address yet. Never assigned to an interface.

Next steps

This is an introduction. SLAAC, DHCPv6 and dual-stack design build on these ideas.

Lesson 11 of 11

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