Ipv6 Overview - Learning Module

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IPv6 Address Notation: Reading and Writing 128-Bit Addresses

The Challenge of Representing 128 Bits

When you first encounter an IPv6 address like 2001:0db8:85a3:0000:0000:8a2e:0370:7334, it can seem intimidating compared to the familiar 192.168.1.1 of IPv4. The address is longer, uses hexadecimal, and features colons as delimiters.

But this complexity is necessary. IPv4's dotted-decimal notation represents 32 bits using four 8-bit decimal numbers (0-255 each). If we tried to extend this to 128 bits, we'd have sixteen numbers—unwieldy and error-prone.

Instead, IPv6 uses a more compact hexadecimal representation: eight groups of four hex digits, separated by colons. And importantly, IPv6 includes abbreviation rules that dramatically shorten most real-world addresses.

What You Will Learn

By the end of this page, you will understand the complete IPv6 address notation system—from full form to maximum abbreviation. You'll be able to read, write, and mentally parse any IPv6 address, and you'll understand how prefix notation works for network designations.

The Full (Canonical) Form

In its complete, unabbreviated form, an IPv6 address consists of:

8 groups (also called hextets, pieces, or blocks)
4 hexadecimal digits per group
Colon (:) separating each group
Total: 32 hex digits representing 128 bits

Structure Breakdown

Full IPv6 Address:

    2001:0db8:85a3:0000:0000:8a2e:0370:7334
    |    |    |    |    |    |    |    |
    └────┴────┴────┴────┴────┴────┴────┴── 8 groups
         
    Group 1: 2001 = 0010 0000 0000 0001 (16 bits)
    Group 2: 0db8 = 0000 1101 1011 1000 (16 bits)
    Group 3: 85a3 = 1000 0101 1010 0011 (16 bits)
    Group 4: 0000 = 0000 0000 0000 0000 (16 bits)
    Group 5: 0000 = 0000 0000 0000 0000 (16 bits)
    Group 6: 8a2e = 1000 1010 0010 1110 (16 bits)
    Group 7: 0370 = 0000 0011 0111 0000 (16 bits)
    Group 8: 7334 = 0111 0011 0011 0100 (16 bits)
                                   └── Total: 128 bits

IPv4 vs IPv6 Notation Comparison
Aspect	IPv4	IPv6
Base	Decimal (0-9)	Hexadecimal (0-9, a-f)
Delimiter	Period (.)	Colon (:)
Groups	4 octets	8 hextets
Bits per group	8 bits (0-255)	16 bits (0000-ffff)
Total bits	32	128
Characters (max)	15 (e.g., 255.255.255.255)	39 (e.g., full form)
Case sensitivity	N/A	Case insensitive (a=A)

Hexadecimal Review

Since IPv6 uses hexadecimal, let's ensure this is clear:

Hex:     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
Decimal: 0  1  2  3  4  5  6  7  8  9  10 11 12 13 14 15
Binary:  0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

Each hex digit represents exactly 4 bits. This makes binary-to-hex conversion straightforward—far easier than decimal conversion.

Why Hexadecimal?

Compact: 32 hex digits vs 128 binary digits
Direct binary mapping: Each hex digit = 4 bits exactly
Readable: More meaningful groupings than decimal
Standard in networking: MAC addresses, memory addresses, etc.
Case-insensitive: 2001:DB8 and 2001:db8 are equivalent

Case Conventions

While uppercase and lowercase are interchangeable, RFC 5952 recommends lowercase for consistency. Most modern systems output lowercase by default. When documenting or configuring, choose one case and stick with it to avoid confusion.

Reading Hexadecimal Groups

Each 16-bit group (hextet) in an IPv6 address can represent values from 0000 to ffff (0 to 65535 in decimal). Understanding how to read these groups is essential.

Value Range Examples

0000 = 0 (minimum value)
0001 = 1
000f = 15
0010 = 16
00ff = 255
0100 = 256
0fff = 4095
1000 = 4096
ffff = 65535 (maximum value)

Common Patterns You'll Encounter

Common Hextet Patterns in IPv6 Addresses
Pattern	Meaning	Example Address
2001	Global unicast documentation prefix	2001:db8::1
2607	Common North American allocation	2607:f8b0::1 (Google)
2a00-2a07	European allocations	2a00:1450::1 (Google EU)
fe80	Link-local prefix	fe80::1
fd00	Unique local (private) prefix	fd00::1
ff02	Link-local multicast	ff02::1 (all-nodes)
0000	Zero group (often abbreviated)	2001:db8::1
ffff	All ones (appears in special addresses)	::ffff:192.0.2.1

Converting Between Hex and Decimal

While you rarely need to convert IPv6 groups to decimal, understanding the process is valuable:

Hextet: 8a2e

Position values:
8    a    2    e
×    ×    ×    ×
4096 256  16   1

Calculation:
(8 × 4096) + (10 × 256) + (2 × 16) + (14 × 1)
= 32768    + 2560      + 32      + 14
= 35374

8a2e in hexadecimal = 35374 in decimal

Practical Tip: For quick mental math, memorize that:

1000 hex = 4096 decimal
0100 hex = 256 decimal
0010 hex = 16 decimal

These round numbers make estimation easy.

Recognizing Address Types by Prefix

With practice, you'll instantly recognize address types by their first hextet: 2xxx/3xxx = global unicast, fe80 = link-local, fdxx = unique local, ff0x = multicast. This pattern recognition becomes automatic and speeds troubleshooting.

Zone IDs and Scope

IPv6 introduces a concept not present in IPv4: scope and zone identifiers. These become important when working with link-local addresses.

The Problem

Link-local addresses (fe80::/10) are only valid on a single network segment. A host with multiple interfaces might have the same link-local address (fe80::1) on each interface. When you want to reach fe80::1, which interface should be used?

The Solution: Zone ID (% Syntax)

IPv6 addresses can include a zone identifier suffix:

fe80::1%eth0      (Linux interface name)
fe80::1%12        (Windows interface index)
fe80::1%en0       (macOS interface name)

The zone ID (after %) specifies which interface or link the address refers to.

Zone ID Formats by Operating System
Operating System	Zone ID Format	Example	How to Find
Linux	Interface name	fe80::1%eth0	`ip link show`
Windows	Interface index	fe80::1%12	`netsh interface ipv6 show interfaces`
macOS	Interface name	fe80::1%en0	`ifconfig`
BSD	Interface name	fe80::1%em0	`ifconfig`

When Zone IDs Are Required

# Pinging a link-local address WITHOUT zone ID:
$ ping fe80::1
ping: connect: Invalid argument    # Error! Which interface?

# Pinging WITH zone ID:
$ ping fe80::1%eth0
PING fe80::1%eth0: 56 data bytes
64 bytes from fe80::1%eth0: icmp_seq=1 ttl=64 time=0.5ms

Address Scopes in IPv6

IPv6 defines several address scopes:

Interface-Local (Scope 1): Loopback only (::1)
Link-Local (Scope 2): Single link (fe80::/10)
Admin-Local (Scope 4): Administratively defined boundary
Site-Local (Scope 5): Deprecated, was similar to private networks
Organization-Local (Scope 8): Multiple sites in organization
Global (Scope E): Internet-wide (2000::/3)

Multicast addresses encode scope in the 4th nibble:

ff01::1  Interface-local all-nodes
ff02::1  Link-local all-nodes
ff05::1  Site-local all-nodes (deprecated)
ff0e::1  Global all-nodes

Zone IDs in URLs

When using link-local addresses in URLs, the % character must be URL-encoded as %25. For example: http://[fe80::1%25eth0]/page.html. This can cause confusion in scripts and configurations—watch for encoding issues.

Prefix Notation (/Length)

Just like IPv4's CIDR notation, IPv6 uses prefix length notation to indicate network boundaries.

Format: address/prefix-length

The prefix length (a number from 0 to 128) indicates how many bits from the left constitute the network portion.

Common Prefix Lengths

Common IPv6 Prefix Lengths
Prefix Length	Usage	Number of Addresses	Example
/128	Single host	1	::1/128 (loopback)
/127	Point-to-point link (RFC 6164)	2	2001:db8::/127
/126	Point-to-point (IPv4 style)	4	Used on some router links
/64	Standard subnet	2^64 (~18 quintillion)	2001:db8:1:1::/64
/56	Home allocation	256 /64 subnets	2001:db8:1::/56
/48	Site allocation	65,536 /64 subnets	2001:db8:1234::/48
/32	ISP allocation	65,536 /48s	2001:db8::/32
/16	Large registry block	~4 billion /48s	2001::/16
/3	Global unicast range	1/8 of total space	2000::/3

Visualizing Prefix Boundaries

IPv6 Address: 2001:0db8:85a3:0001:0000:8a2e:0370:7334/64

Binary representation (showing /64 boundary):

     Network Prefix (64 bits)                  Interface ID (64 bits)
 ┌──────────────────────────────────────┬──────────────────────────────────────┐
 │ 0010000000000001 0000110110111000    │ 0000000000000000 1000101000101110    │
 │ 1000010110100011 0000000000000001    │ 0000001101110000 0111001100110100    │
 └──────────────────────────────────────┴──────────────────────────────────────┘
   2001:0db8:85a3:0001                    0000:8a2e:0370:7334

   ▲                                      ▲
   │                                      │
   Network: 2001:db8:85a3:1::/64         Host portion

Calculating the Network Address

To find the network address from a host address:

Convert to binary
Keep the first N bits (prefix length)
Zero out the remaining bits
Convert back to hexadecimal

Example: 2001:db8:85a3:1:1234:5678:9abc:def0/48

Original: 2001:0db8:85a3:0001:1234:5678:9abc:def0
                          ↑
                    /48 boundary (after 3rd hextet)

Network:  2001:0db8:85a3:0000:0000:0000:0000:0000
       =  2001:db8:85a3::/48

Prefix Length vs Subnet Mask

Unlike IPv4, IPv6 does not use subnet mask notation (like 255.255.255.0). Prefix length notation (/64) is the only standard method. If someone asks for an IPv6 'subnet mask,' they mean the prefix length.

Representing Special Addresses

Certain IPv6 addresses have special meanings and standardized representations. Understanding these is essential for network operations.

The Loopback Address

Full form:   0000:0000:0000:0000:0000:0000:0000:0001
Abbreviated: ::1

Equivalent to IPv4's 127.0.0.1
Used for: Testing local TCP/IP stack

The Unspecified Address

Full form:   0000:0000:0000:0000:0000:0000:0000:0000
Abbreviated: ::

Equivalent to IPv4's 0.0.0.0
Used for: Source address before configuration
         Listening on all addresses
         Default route destination

IPv4-Mapped IPv6 Addresses

These represent IPv4 addresses within the IPv6 format:

Format: ::ffff:IPv4-address

Examples:
::ffff:192.0.2.1        (using dotted-decimal)
::ffff:c000:0201        (pure hex form)

Used by: Dual-stack applications
         IPv6 sockets accepting IPv4 connections

Special IPv6 Address Representations
Address	Full Form	Abbreviated	Purpose
Loopback	0:0:0:0:0:0:0:1	::1	Local host testing
Unspecified	0:0:0:0:0:0:0:0	::	No address / any address
IPv4-mapped	0:0:0:0:0:ffff:IPv4	::ffff:a.b.c.d	IPv4 in IPv6 format
IPv4-compatible	0:0:0:0:0:0:a.b.c.d	::a.b.c.d	Deprecated (don't use)
Link-local all-nodes	ff02:0:0:0:0:0:0:1	ff02::1	Multicast to all hosts on link
Link-local all-routers	ff02:0:0:0:0:0:0:2	ff02::2	Multicast to all routers on link
Documentation	2001:db8::/32	2001:db8::x	Examples in documentation

Documentation Addresses (2001:db8::/32)

RFC 3849 reserves 2001:db8::/32 for documentation purposes:

2001:db8::1
2001:db8:1234:5678:9abc:def0:1234:5678
2001:db8:cafe::/48

These addresses will never be allocated for real use. Always use them in:

Technical documentation
Examples and tutorials
Configuration templates
Test environments

Using real addresses in documentation can cause accidental traffic to those addresses—use 2001:db8:: instead.

Link-Local Address Format

Prefix: fe80::/10
Bits 11-64: Must be zero
Bits 65-128: Interface identifier

Result: fe80:0000:0000:0000:xxxx:xxxx:xxxx:xxxx
        fe80::xxxx:xxxx:xxxx:xxxx

Example: fe80::1a2b:3cff:fe4d:5e6f

Don't Use Real Addresses in Documentation

If you publish documentation using a real IPv6 address (like 2607:f8b0::1 - Google), readers might accidentally send traffic to that address. Always use 2001:db8::/32 for examples. This is the networking equivalent of using 'example.com' instead of real domain names.

Mixed Notation (IPv4 in IPv6)

IPv6 supports a special notation that embeds IPv4 addresses in dotted-decimal format within the IPv6 address.

Format: The last 32 bits can be written in IPv4 dotted-decimal:

General:  prefix:IPv4-address
Example:  ::ffff:192.0.2.1

Equivalent pure hex: ::ffff:c000:0201

     c000 = 192 × 256 + 0 = 49152 (c0.00 hex)
     0201 = 2 × 256 + 1 = 513 (02.01 hex)

Where Mixed Notation Appears

Mixed (IPv4-in-IPv6) Notation Uses
Address Type	Format	Example	Usage
IPv4-mapped	::ffff:a.b.c.d	::ffff:192.168.1.1	Dual-stack socket representation
IPv4-compatible	::a.b.c.d	::192.168.1.1	Deprecated, historical only
NAT64/DNS64	64:ff9b::a.b.c.d	64:ff9b::8.8.8.8	IPv4/IPv6 translation
ISATAP	prefix::0:5efe:a.b.c.d	2001:db8::5efe:10.0.0.1	Transition tunneling
6to4	2002:IPv4::/48	2002:c000:0201::/48	Automatic tunneling

Understanding IPv4-Mapped Addresses

When an IPv6-enabled application receives a connection from an IPv4 address, the operating system presents it as:

IPv4 source: 192.0.2.100:5000
Appears as:  ::ffff:192.0.2.100 port 5000

This allows IPv6-native applications to accept IPv4 connections transparently on dual-stack systems.

Example: Server Socket

# Python server listening on IPv6 socket
import socket

server = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
server.bind(('::', 8080))  # Bind to all IPv6 addresses
server.listen(5)

conn, addr = server.accept()
print(f"Connection from: {addr}")

# If IPv4 client connects from 192.168.1.100:
# Output: Connection from: ('::ffff:192.168.1.100', 54321, 0, 0)

NAT64 Translation Prefix

Well-known prefix: 64:ff9b::/96

IPv4 address: 8.8.8.8 (Google DNS)
NAT64 form:   64:ff9b::8.8.8.8
           or 64:ff9b::0808:0808

IPv6-only clients can reach 8.8.8.8 through NAT64 gateway
using this translated address.

When to Use Mixed Notation

Use mixed notation when the IPv4 component is meaningful—like in transition mechanisms or when logging dual-stack connections. For pure IPv6 addresses, always use the standard colon-hex format. Avoid mixed notation for addresses that have no IPv4 relationship.

URL and Configuration Syntax

Using IPv6 addresses in URLs and configuration files requires special handling because colons already have meaning in these contexts.

The Problem

In a URL like http://host:port/path, the colon separates host from port. But IPv6 addresses contain colons:

Ambiguous: http://2001:db8::1:8080/   # Is 8080 the port or part of IP?

The Solution: Square Brackets

Wrap IPv6 addresses in square brackets:

Correct: http://[2001:db8::1]:8080/

Format: protocol://[IPv6-address]:port/path

IPv6 in URLs and Configuration
Context	Syntax	Example
HTTP URL	http://[address]:port/	http://[2001:db8::1]:80/
HTTPS URL	https://[address]:port/	https://[2001:db8::1]:443/
SSH	user@[address]	ssh user@[2001:db8::1]
FTP URL	ftp://[address]/	ftp://[2001:db8::1]/
SFTP	sftp://[address]/	sftp://[2001:db8::1]/
SCP		scp file [2001:db8::1]:/tmp/
rsync		rsync -av [2001:db8::1]::module/
PostgreSQL	host=[address]	postgresql://[::1]:5432/db
DNS zone	AAAA record	host IN AAAA 2001:db8::1

Link-Local Addresses with Zone IDs in URLs

Link-local addresses require zone IDs, which contain % characters that must be URL-encoded:

Address:    fe80::1%eth0
URL-encoded fe80::1%25eth0
Full URL:   http://[fe80::1%25eth0]:8080/
            └──────────────────────────────
                   %25 = URL-encoded %

Configuration File Examples

# Nginx configuration
server {
    listen [::]:80;               # Listen on all IPv6 addresses
    listen [2001:db8::1]:80;      # Listen on specific IPv6
    server_name example.com;
}

# Apache configuration
Listen [::]:80
Listen [2001:db8::1]:443

<VirtualHost [2001:db8::1]:443>
    ServerName secure.example.com
</VirtualHost>

# SSH config (~/.ssh/config)
Host myserver
    HostName 2001:db8::1
    # Note: No brackets in ssh_config HostName

Command Line Tools

# ping (varies by OS)
ping6 2001:db8::1                    # Linux (some systems)
ping -6 2001:db8::1                  # Linux (systemd)
ping 2001:db8::1                     # Modern Linux (auto-detect)
ping -6 2001:db8::1                  # macOS, Windows

# curl
curl http://[2001:db8::1]:8080/
curl -g "http://[2001:db8::1]:8080/" # -g disables glob parsing

# wget
wget http://[2001:db8::1]/file.txt

# netcat
nc -6 2001:db8::1 80

Shell Escaping

Some shells interpret square brackets as glob patterns. Use quotes or the -g flag (curl) to prevent expansion. If you see 'no matches found' errors, quoting is likely the solution: curl "http://[2001:db8::1]/"

Summary: IPv6 Address Notation

IPv6 address notation uses hexadecimal and colons to represent 128-bit addresses in a relatively compact form. Let's consolidate the key concepts:

Key Takeaways

•Full form uses 8 groups of 4 hex digits — 32 hexadecimal characters total, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
•Hexadecimal provides direct binary mapping — Each hex digit represents exactly 4 bits, simplifying binary analysis.
•Case is insensitive but lowercase is preferred — RFC 5952 recommends lowercase for consistency.
•Zone IDs (%interface) are required for link-local — Specify which interface when using fe80:: addresses.
•Prefix notation (/length) indicates network size — Similar to CIDR, /64 is the standard subnet size.
•Special addresses have defined representations — ::1 for loopback, :: for unspecified, 2001:db8:: for documentation.
•Mixed notation embeds IPv4 in last 32 bits — Used for IPv4-mapped (::ffff:a.b.c.d) and transition mechanisms.
•Square brackets enclose IPv6 in URLs — Required to separate address from port (http://[address]:port/).

What's Next

Now that we can read and write IPv6 addresses, we'll learn the abbreviation rules that dramatically shorten most addresses. These rules make IPv6 much more practical for daily use—transforming 2001:0db8:0000:0000:0000:0000:0000:0001 into the much friendlier 2001:db8::1.

Page Complete

You can now read and write IPv6 addresses in their full form, understand special address representations, use zone IDs for link-local communication, and properly format addresses in URLs and configuration files. Next, we'll master the abbreviation rules that make IPv6 manageable in practice.

IPv6 Address Notation: Reading and Writing 128-Bit Addresses

The Challenge of Representing 128 Bits

What You Will Learn

The Full (Canonical) Form

In its complete, unabbreviated form, an IPv6 address consists of:

8 groups (also called hextets, pieces, or blocks)
4 hexadecimal digits per group
Colon (:) separating each group
Total: 32 hex digits representing 128 bits

Structure Breakdown

Full IPv6 Address:

    2001:0db8:85a3:0000:0000:8a2e:0370:7334
    |    |    |    |    |    |    |    |
    └────┴────┴────┴────┴────┴────┴────┴── 8 groups
         
    Group 1: 2001 = 0010 0000 0000 0001 (16 bits)
    Group 2: 0db8 = 0000 1101 1011 1000 (16 bits)
    Group 3: 85a3 = 1000 0101 1010 0011 (16 bits)
    Group 4: 0000 = 0000 0000 0000 0000 (16 bits)
    Group 5: 0000 = 0000 0000 0000 0000 (16 bits)
    Group 6: 8a2e = 1000 1010 0010 1110 (16 bits)
    Group 7: 0370 = 0000 0011 0111 0000 (16 bits)
    Group 8: 7334 = 0111 0011 0011 0100 (16 bits)
                                   └── Total: 128 bits

IPv4 vs IPv6 Notation Comparison
Aspect	IPv4	IPv6
Base	Decimal (0-9)	Hexadecimal (0-9, a-f)
Delimiter	Period (.)	Colon (:)
Groups	4 octets	8 hextets
Bits per group	8 bits (0-255)	16 bits (0000-ffff)
Total bits	32	128
Characters (max)	15 (e.g., 255.255.255.255)	39 (e.g., full form)
Case sensitivity	N/A	Case insensitive (a=A)

Hexadecimal Review

Since IPv6 uses hexadecimal, let's ensure this is clear:

Hex:     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
Decimal: 0  1  2  3  4  5  6  7  8  9  10 11 12 13 14 15
Binary:  0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

Each hex digit represents exactly 4 bits. This makes binary-to-hex conversion straightforward—far easier than decimal conversion.

Why Hexadecimal?

Compact: 32 hex digits vs 128 binary digits
Direct binary mapping: Each hex digit = 4 bits exactly
Readable: More meaningful groupings than decimal
Standard in networking: MAC addresses, memory addresses, etc.
Case-insensitive: 2001:DB8 and 2001:db8 are equivalent

Case Conventions

Reading Hexadecimal Groups

Each 16-bit group (hextet) in an IPv6 address can represent values from 0000 to ffff (0 to 65535 in decimal). Understanding how to read these groups is essential.

Value Range Examples

0000 = 0 (minimum value)
0001 = 1
000f = 15
0010 = 16
00ff = 255
0100 = 256
0fff = 4095
1000 = 4096
ffff = 65535 (maximum value)

Common Patterns You'll Encounter

Common Hextet Patterns in IPv6 Addresses
Pattern	Meaning	Example Address
2001	Global unicast documentation prefix	2001:db8::1
2607	Common North American allocation	2607:f8b0::1 (Google)
2a00-2a07	European allocations	2a00:1450::1 (Google EU)
fe80	Link-local prefix	fe80::1
fd00	Unique local (private) prefix	fd00::1
ff02	Link-local multicast	ff02::1 (all-nodes)
0000	Zero group (often abbreviated)	2001:db8::1
ffff	All ones (appears in special addresses)	::ffff:192.0.2.1

Converting Between Hex and Decimal

While you rarely need to convert IPv6 groups to decimal, understanding the process is valuable:

Hextet: 8a2e

Position values:
8    a    2    e
×    ×    ×    ×
4096 256  16   1

Calculation:
(8 × 4096) + (10 × 256) + (2 × 16) + (14 × 1)
= 32768    + 2560      + 32      + 14
= 35374

8a2e in hexadecimal = 35374 in decimal

Practical Tip: For quick mental math, memorize that:

1000 hex = 4096 decimal
0100 hex = 256 decimal
0010 hex = 16 decimal

These round numbers make estimation easy.

Recognizing Address Types by Prefix

Zone IDs and Scope

IPv6 introduces a concept not present in IPv4: scope and zone identifiers. These become important when working with link-local addresses.

The Problem

The Solution: Zone ID (% Syntax)

IPv6 addresses can include a zone identifier suffix:

fe80::1%eth0      (Linux interface name)
fe80::1%12        (Windows interface index)
fe80::1%en0       (macOS interface name)

The zone ID (after %) specifies which interface or link the address refers to.

Zone ID Formats by Operating System
Operating System	Zone ID Format	Example	How to Find
Linux	Interface name	fe80::1%eth0	`ip link show`
Windows	Interface index	fe80::1%12	`netsh interface ipv6 show interfaces`
macOS	Interface name	fe80::1%en0	`ifconfig`
BSD	Interface name	fe80::1%em0	`ifconfig`

When Zone IDs Are Required

# Pinging a link-local address WITHOUT zone ID:
$ ping fe80::1
ping: connect: Invalid argument    # Error! Which interface?

# Pinging WITH zone ID:
$ ping fe80::1%eth0
PING fe80::1%eth0: 56 data bytes
64 bytes from fe80::1%eth0: icmp_seq=1 ttl=64 time=0.5ms

Address Scopes in IPv6

IPv6 defines several address scopes:

Interface-Local (Scope 1): Loopback only (::1)
Link-Local (Scope 2): Single link (fe80::/10)
Admin-Local (Scope 4): Administratively defined boundary
Site-Local (Scope 5): Deprecated, was similar to private networks
Organization-Local (Scope 8): Multiple sites in organization
Global (Scope E): Internet-wide (2000::/3)

Multicast addresses encode scope in the 4th nibble:

ff01::1  Interface-local all-nodes
ff02::1  Link-local all-nodes
ff05::1  Site-local all-nodes (deprecated)
ff0e::1  Global all-nodes

Zone IDs in URLs

Prefix Notation (/Length)

Just like IPv4's CIDR notation, IPv6 uses prefix length notation to indicate network boundaries.

Format: address/prefix-length

The prefix length (a number from 0 to 128) indicates how many bits from the left constitute the network portion.

Common Prefix Lengths

Common IPv6 Prefix Lengths
Prefix Length	Usage	Number of Addresses	Example
/128	Single host	1	::1/128 (loopback)
/127	Point-to-point link (RFC 6164)	2	2001:db8::/127
/126	Point-to-point (IPv4 style)	4	Used on some router links
/64	Standard subnet	2^64 (~18 quintillion)	2001:db8:1:1::/64
/56	Home allocation	256 /64 subnets	2001:db8:1::/56
/48	Site allocation	65,536 /64 subnets	2001:db8:1234::/48
/32	ISP allocation	65,536 /48s	2001:db8::/32
/16	Large registry block	~4 billion /48s	2001::/16
/3	Global unicast range	1/8 of total space	2000::/3

Visualizing Prefix Boundaries

IPv6 Address: 2001:0db8:85a3:0001:0000:8a2e:0370:7334/64

Binary representation (showing /64 boundary):

     Network Prefix (64 bits)                  Interface ID (64 bits)
 ┌──────────────────────────────────────┬──────────────────────────────────────┐
 │ 0010000000000001 0000110110111000    │ 0000000000000000 1000101000101110    │
 │ 1000010110100011 0000000000000001    │ 0000001101110000 0111001100110100    │
 └──────────────────────────────────────┴──────────────────────────────────────┘
   2001:0db8:85a3:0001                    0000:8a2e:0370:7334

   ▲                                      ▲
   │                                      │
   Network: 2001:db8:85a3:1::/64         Host portion

Calculating the Network Address

To find the network address from a host address:

Convert to binary
Keep the first N bits (prefix length)
Zero out the remaining bits
Convert back to hexadecimal

Example: 2001:db8:85a3:1:1234:5678:9abc:def0/48

Original: 2001:0db8:85a3:0001:1234:5678:9abc:def0
                          ↑
                    /48 boundary (after 3rd hextet)

Network:  2001:0db8:85a3:0000:0000:0000:0000:0000
       =  2001:db8:85a3::/48

Prefix Length vs Subnet Mask

Representing Special Addresses

Certain IPv6 addresses have special meanings and standardized representations. Understanding these is essential for network operations.

The Loopback Address

Full form:   0000:0000:0000:0000:0000:0000:0000:0001
Abbreviated: ::1

Equivalent to IPv4's 127.0.0.1
Used for: Testing local TCP/IP stack

The Unspecified Address

Full form:   0000:0000:0000:0000:0000:0000:0000:0000
Abbreviated: ::

Equivalent to IPv4's 0.0.0.0
Used for: Source address before configuration
         Listening on all addresses
         Default route destination

IPv4-Mapped IPv6 Addresses

These represent IPv4 addresses within the IPv6 format:

Format: ::ffff:IPv4-address

Examples:
::ffff:192.0.2.1        (using dotted-decimal)
::ffff:c000:0201        (pure hex form)

Used by: Dual-stack applications
         IPv6 sockets accepting IPv4 connections

Special IPv6 Address Representations
Address	Full Form	Abbreviated	Purpose
Loopback	0:0:0:0:0:0:0:1	::1	Local host testing
Unspecified	0:0:0:0:0:0:0:0	::	No address / any address
IPv4-mapped	0:0:0:0:0:ffff:IPv4	::ffff:a.b.c.d	IPv4 in IPv6 format
IPv4-compatible	0:0:0:0:0:0:a.b.c.d	::a.b.c.d	Deprecated (don't use)
Link-local all-nodes	ff02:0:0:0:0:0:0:1	ff02::1	Multicast to all hosts on link
Link-local all-routers	ff02:0:0:0:0:0:0:2	ff02::2	Multicast to all routers on link
Documentation	2001:db8::/32	2001:db8::x	Examples in documentation

Documentation Addresses (2001:db8::/32)

RFC 3849 reserves 2001:db8::/32 for documentation purposes:

2001:db8::1
2001:db8:1234:5678:9abc:def0:1234:5678
2001:db8:cafe::/48

These addresses will never be allocated for real use. Always use them in:

Technical documentation
Examples and tutorials
Configuration templates
Test environments

Using real addresses in documentation can cause accidental traffic to those addresses—use 2001:db8:: instead.

Link-Local Address Format

Prefix: fe80::/10
Bits 11-64: Must be zero
Bits 65-128: Interface identifier

Result: fe80:0000:0000:0000:xxxx:xxxx:xxxx:xxxx
        fe80::xxxx:xxxx:xxxx:xxxx

Example: fe80::1a2b:3cff:fe4d:5e6f

Don't Use Real Addresses in Documentation

Mixed Notation (IPv4 in IPv6)

IPv6 supports a special notation that embeds IPv4 addresses in dotted-decimal format within the IPv6 address.

Format: The last 32 bits can be written in IPv4 dotted-decimal:

General:  prefix:IPv4-address
Example:  ::ffff:192.0.2.1

Equivalent pure hex: ::ffff:c000:0201

     c000 = 192 × 256 + 0 = 49152 (c0.00 hex)
     0201 = 2 × 256 + 1 = 513 (02.01 hex)

Where Mixed Notation Appears

Mixed (IPv4-in-IPv6) Notation Uses
Address Type	Format	Example	Usage
IPv4-mapped	::ffff:a.b.c.d	::ffff:192.168.1.1	Dual-stack socket representation
IPv4-compatible	::a.b.c.d	::192.168.1.1	Deprecated, historical only
NAT64/DNS64	64:ff9b::a.b.c.d	64:ff9b::8.8.8.8	IPv4/IPv6 translation
ISATAP	prefix::0:5efe:a.b.c.d	2001:db8::5efe:10.0.0.1	Transition tunneling
6to4	2002:IPv4::/48	2002:c000:0201::/48	Automatic tunneling

Understanding IPv4-Mapped Addresses

When an IPv6-enabled application receives a connection from an IPv4 address, the operating system presents it as:

IPv4 source: 192.0.2.100:5000
Appears as:  ::ffff:192.0.2.100 port 5000

This allows IPv6-native applications to accept IPv4 connections transparently on dual-stack systems.

Example: Server Socket

# Python server listening on IPv6 socket
import socket

server = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
server.bind(('::', 8080))  # Bind to all IPv6 addresses
server.listen(5)

conn, addr = server.accept()
print(f"Connection from: {addr}")

# If IPv4 client connects from 192.168.1.100:
# Output: Connection from: ('::ffff:192.168.1.100', 54321, 0, 0)

NAT64 Translation Prefix

Well-known prefix: 64:ff9b::/96

IPv4 address: 8.8.8.8 (Google DNS)
NAT64 form:   64:ff9b::8.8.8.8
           or 64:ff9b::0808:0808

IPv6-only clients can reach 8.8.8.8 through NAT64 gateway
using this translated address.

When to Use Mixed Notation

URL and Configuration Syntax

Using IPv6 addresses in URLs and configuration files requires special handling because colons already have meaning in these contexts.

The Problem

In a URL like http://host:port/path, the colon separates host from port. But IPv6 addresses contain colons:

Ambiguous: http://2001:db8::1:8080/   # Is 8080 the port or part of IP?

The Solution: Square Brackets

Wrap IPv6 addresses in square brackets:

Correct: http://[2001:db8::1]:8080/

Format: protocol://[IPv6-address]:port/path

IPv6 in URLs and Configuration
Context	Syntax	Example
HTTP URL	http://[address]:port/	http://[2001:db8::1]:80/
HTTPS URL	https://[address]:port/	https://[2001:db8::1]:443/
SSH	user@[address]	ssh user@[2001:db8::1]
FTP URL	ftp://[address]/	ftp://[2001:db8::1]/
SFTP	sftp://[address]/	sftp://[2001:db8::1]/
SCP		scp file [2001:db8::1]:/tmp/
rsync		rsync -av [2001:db8::1]::module/
PostgreSQL	host=[address]	postgresql://[::1]:5432/db
DNS zone	AAAA record	host IN AAAA 2001:db8::1

Link-Local Addresses with Zone IDs in URLs

Link-local addresses require zone IDs, which contain % characters that must be URL-encoded:

Address:    fe80::1%eth0
URL-encoded fe80::1%25eth0
Full URL:   http://[fe80::1%25eth0]:8080/
            └──────────────────────────────
                   %25 = URL-encoded %

Configuration File Examples

# Nginx configuration
server {
    listen [::]:80;               # Listen on all IPv6 addresses
    listen [2001:db8::1]:80;      # Listen on specific IPv6
    server_name example.com;
}

# Apache configuration
Listen [::]:80
Listen [2001:db8::1]:443

<VirtualHost [2001:db8::1]:443>
    ServerName secure.example.com
</VirtualHost>

# SSH config (~/.ssh/config)
Host myserver
    HostName 2001:db8::1
    # Note: No brackets in ssh_config HostName

Command Line Tools

# ping (varies by OS)
ping6 2001:db8::1                    # Linux (some systems)
ping -6 2001:db8::1                  # Linux (systemd)
ping 2001:db8::1                     # Modern Linux (auto-detect)
ping -6 2001:db8::1                  # macOS, Windows

# curl
curl http://[2001:db8::1]:8080/
curl -g "http://[2001:db8::1]:8080/" # -g disables glob parsing

# wget
wget http://[2001:db8::1]/file.txt

# netcat
nc -6 2001:db8::1 80

Shell Escaping

Summary: IPv6 Address Notation

IPv6 address notation uses hexadecimal and colons to represent 128-bit addresses in a relatively compact form. Let's consolidate the key concepts:

Key Takeaways

•Full form uses 8 groups of 4 hex digits — 32 hexadecimal characters total, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
•Hexadecimal provides direct binary mapping — Each hex digit represents exactly 4 bits, simplifying binary analysis.
•Case is insensitive but lowercase is preferred — RFC 5952 recommends lowercase for consistency.
•Zone IDs (%interface) are required for link-local — Specify which interface when using fe80:: addresses.
•Prefix notation (/length) indicates network size — Similar to CIDR, /64 is the standard subnet size.
•Special addresses have defined representations — ::1 for loopback, :: for unspecified, 2001:db8:: for documentation.
•Mixed notation embeds IPv4 in last 32 bits — Used for IPv4-mapped (::ffff:a.b.c.d) and transition mechanisms.
•Square brackets enclose IPv6 in URLs — Required to separate address from port (http://[address]:port/).

What's Next

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