Understanding IPv6 Transition Technologies

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Jun 30, 2012 (5 years and 3 months ago)

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Understanding IPv6
Transition Technologies
Joe
Davies
Principal Technical
Writer
Windows
Server User Assistance
Microsoft
Corporation
Presented at:
Seattle
Windows Networking User Group
September 2, 2009
Agenda

Transition Terms and Concepts

ISATAP

6to4

Teredo

IP
-
HTTPS
Transition Terms and
Concepts

Node Types

Transition Mechanisms
Node Types

IPv4
-
only node

IPv6
-
only node

IPv4 node

IPv6 node

IPv6/IPv4 node
Transition Mechanisms

Using both IPv4 and IPv6

IPv6
over IPv4
tunneling

DNS infrastructure
Application
Layer
TCP/UDP
IPv6
Network
Interface Layer
TCP/UDP
IPv4
Dual Stack Architecture

Implementation
in Windows XP,
Windows CE, and Windows Server 2003
Application
Layer
Transport Layer (TCP/UDP)
IPv6
Network
Interface Layer
IPv4
Dual IP Layer Architecture

Next
-
Generation TCP/IP stack in Windows
Vista, Windows 7, Windows Server 2008,
Windows Server 2008 R2
Types of Packets with a Dual IP Layer
Architecture
Application
Layer
Transport Layer (TCP/UDP)
IPv6
Network
Interface Layer
IPv4
IPv6
IPv4
IPv6 over IPv4
IPv6 Header
Extension
Headers
Upper Layer Protocol
Data Unit
IPv6 Header
Extension
Headers
Upper Layer Protocol
Data Unit
IPv4 Header
IPv6 Packet
IPv4 Packet
IPv6 over IPv4 Tunneling
IPv4 header Protocol field set to 41
Types of Tunnels

Configured

Manual configuration of tunnel endpoints

Example:

Use
netsh interface ipv6 add v6v4tunnel
command

Need to add routes using the tunnel interface

Automatic

Tunnel endpoints determined from use of routes,
logical tunnel interfaces, and destination IPv6
addresses

Examples:

ISATAP, 6to4
,
Teredo, IP
-
HTTPS
DNS Infrastructure

Address records

A records for IPv4 nodes

AAAA records for IPv6 nodes

Pointer records

PTR records in IN
-
ADDR.ARPA domain
for IPv4 nodes

PTR records in
IP6.ARPA
domain for
IPv6 nodes
Windows Transition Technologies

ISATAP

Unicast communication across an
IPv4
-
only
intranet

6to4

Unicast communication across the IPv4 Internet

Teredo

Unicast communication across the IPv4 Internet
over
NATs

IP
-
HTTPS

Unicast communication across the IPv4 Internet
over firewalls and proxies
Use of Transition Technologies

Host receives native Router Advertisement

Uses native IPv6

Host obtains ISATAP Router Advertisement

Uses ISATAP

Host has public IPv4 address

Uses 6to4

Host has private IPv4 address

Uses
Teredo

DirectAccess client cannot connect with 6to4
or Teredo

Uses IP
-
HTTPS
Questions?
ISATAP Overview

Address assignment and automatic
tunneling technology for unicast IPv6 traffic
between IPv6/IPv4 nodes across an
IPv4
-
only intranet

ISATAP addresses:

64
-
bitUnicastPrefix
:0:5efe:
w.x.y.z

w.x.y.z
is a private IPv4 address

64
-
bitUnicastPrefix
:200:5efe:
w.x.y.z

w.x.y.z
is a public IPv4 address

Example: fe80::200:5efe:157.59.137.133

ISATAP
treats an IPv4 infrastructure as a
single link
ISATAP Components
ISATAP host
IPv6 over IPv4 traffic
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
IPv6 traffic
Obtaining an ISATAP Prefix
ISATAP host
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
DNS server
IPv6 tunneled with IPv4

1. DNS query for “ISATAP”

2. IPv4
-
encapsulated router solicitation

3. IPv4
-
encapsulated router advertisement
IPv4 traffic
ISATAP Addressing
ISATAP host
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
192.168.47.99
192.168.71.209
Example prefix: 3FFE:2900:D005:7::/64
3FFE:2900:D005:7:0:5EFE:192.168.71.209
3FFE:2900:D005:7:0:5EFE:192.168.47.99
ISATAP interface
ID (private IPv4 address):
::0:5EFE:
w.x.y.z
ISATAP Routing
ISATAP host
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
Example prefix: 3FFE:2900:D005:7::/64
Routes:
3FFE:2900:D005:7::/64
on link
via ISATAP interface
::/0 to ISATAP router via ISATAP interface
Routes:
3FFE:2900:D005:7::/64
on link
via
ISATAP interface
::/0 to ISATAP router via ISATAP
interface
Routes:
3FFE:2900:D005:7::/64
on link
via ISATAP
interface
::/0 to
IPv6 intranet
via
LAN
interface
Routes:
3FFE:2900:D005:7::/64 to
ISATAP router
ISATAP Host to ISATAP Host
ISATAP host A
IPv4
-
only intranet
ISATAP host B
IPv6 intranet
ISATAP router
Example prefix: 3FFE:2900:D005:7::/64
IPv4 Header:
Destination Address:
192.168.71.209
Source Address: 192.168.47.99
IPv6 Header:
Destination Address:
3FFE:2900:D005:7:0:5EFE:192.168.71.209
Source Address: 3FFE:2900:D005:7:0:5EFE:192.168.47.99
Routes:
3FFE:2900:D005:7::/64
on link
via ISATAP interface
::/0 to ISATAP router
IPv6 tunneled over IPv4
Native IPv6 traffic
3FFE:2900:D005:7:0:5EFE:192.168.71.209
3FFE:2900:D005:7:0:5EFE:192.168.47.99
ISATAP Host to IPv6 Host (Pt 1)
ISATAP host
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
Example prefix: 3FFE:2900:D005:7::/64
Routes:
3FFE:2900:D005:7::/64
on link
::/0 to ISATAP router via ISATAP interface
IPv4 Header:
Destination Address: 10.0.0.1
Source Address: 192.168.47.99
IPv6 Header:
Destination Address:
3FFE:2900:D005:99:2AA:FF:FE9A:21AC
Source Address: 3FFE:2900:D005:7:0:5EFE:192.168.47.99
10.0.0.1
3FFE:2900:D005:99:2AA:FF:FE9A:21AC
IPv6 tunneled over IPv4
Native IPv6 traffic
3FFE:2900:D005:7:0:5EFE:192.168.47.99
ISATAP Host to IPv6 Host (Pt 2)
ISATAP host
IPv4
-
only intranet
ISATAP host
IPv6 intranet
ISATAP router
Example prefix: 3FFE:2900:D005:7::/64
IPv6 Header:
Destination Address:
3FFE:2900:D005:99:2AA:FF:FE9A:21AC
Source Address: 3FFE:2900:D005:7:0:5EFE:192.168.47.99
3FFE:2900:D005:99:2AA:FF:FE9A:21AC
IPv6 tunneled over IPv4
Native IPv6 traffic
Routes:
3FFE:2900:D005:7::/64
on
link via ISATAP interface
::/0 to
IPv6 intranet
via LAN
interface
Questions?
6to4 Overview

Address assignment and automatic
tunneling technology for unicast traffic
between IPv6/IPv4 nodes across the IPv4
Internet

6to4 address:

WWXX
:
YYZZ
is the colon
-
hexadecimal
representation of
w.x.y.z
, a public IPv4 address

6to4 treats the IPv4 Internet as a single link
Subnet ID
Interface ID
WWXX
:
YYZZ
32 bits
16 bits
64 bits
2002
16 bits
6to4 Components
6to4 router
6to4 host A
IPv6 traffic
IPv6 over IPv4 traffic
IPv4 Internet
6to4 host/router B
IPv6 Internet
6to4 relay
IPv6 traffic
6to4 in Windows

6to4 host/router

If there is a public
IPv4
address, Windows
autoconfigures a 6to4
address

2002:
WWXX
:
YYZZ
::
WWXX
:
YYZZ

Queries 6to4.ipv6.microsoft.com for an IPv4
address of a 6to4
relay

6to4
router

Enable ICS on Internet interface

Windows enables IPv6 routing and advertising of the 6to4
prefix on the private interface

Queries 6to4.ipv6.microsoft.com for an IPv4
address of a 6to4
relay
6to4 Addressing Example
6to4 router
6to4 host A
IPv4 Internet
6to4 host/router B
131.107.0.1
IPv6 Internet
6to4 relay
157.60.0.1
Advertised prefix: 2002:9D3C:1:1::/64
2002:836B:1::836B:1
2002:9D3C:1:1::1
6to4 prefix: 2002:
WWXX
:
YYZZ
::/48
6to4 Routing
6to4 router
6to4 host A
IPv4 Internet
6to4 host/router B
IPv6 Internet
6to4 relay
Routes:
2002::/16
on link
via 6to4 interface
::/0 to 6to4 relay via 6to4 interface
2002:9D3C:1:1::/64
on link
via LAN interface
Routes:
2002::/16
on link
via 6to4 interface
::/0 to IPv6 Internet via LAN interface
Routes:
2002::/16
on link
via 6to4 interface
::/0 to 6to4 relay via 6to4 interface
Routes:
2002:9D3C:1:1::/64
on link
via LAN interface
::/0 to 6to4 router via LAN interface
Routes:
2002::/16 to 6to4 relay
6to4 Host to 6to4 Host (Pt 1)
6to4 router
6to4 host A
2002:9D3C:1:1::1
IPv4 Internet
6to4 host/router B
2002:836B:1::836B:1
IPv6 Internet
6to4 relay
IPv6 Header:
Destination Address: 2002:836B:1::836B:1
Source Address: 2002:9D3C:1:1::1
Routes:
2002:9D3C:1:1::/64 to local subnet
::/0 to 6to4 router via LAN interface
IPv6 tunneled over IPv4
Native IPv6 traffic
6to4 Host to 6to4 Host (Pt 2)
6to4 router
6to4 host A
2002:9D3C:1:1::1
IPv4 Internet
6to4 host/router B
2002:836B:1::836B:1
IPv6 Internet
6to4 relay
Routes:
2002::/16
on link
via 6to4 interface
::/0 to 6to4 relay
2002:9D3C:1:1::/64 to local subnet
IPv4 Header:
Destination Address: 131.107.0.1
Source Address: 157.60.0.1
IPv6 Header:
Destination Address: 2002:836B:1::836B:1
Source Address: 2002:9D3C:1:1::1
IPv6 tunneled over IPv4
Native IPv6 traffic
6to4 Host to IPv6 Host (Pt 1)
6to4 router
6to4 host A
2002:9D3C:1:1::1
IPv4 Internet
6to4 host/router B
IPv6 Internet
6to4 relay
IPv6 Header:
Destination Address:
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
Source Address: 2002:9D3C:1:1::1
Routes:
2002:9D3C:1:1::/64 to local subnet
::/0 to 6to4 router via LAN interface
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
IPv6 tunneled over IPv4
Native IPv6 traffic
6to4 Host to IPv6 Host (Pt 2)
6to4 router
6to4 host A
2002:9D3C:1:1::1
IPv4 Internet
6to4 host/router B
IPv6 Internet
6to4 relay
IPv4 Header:
Destination Address
: 192.88.99.1
Source Address:
157.60.0.1
IPv6 Header:
Destination Address:
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
Source Address: 2002:9D3C:1:1::1
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
IPv6 tunneled over IPv4
Native IPv6 traffic
Routes:
2002::/16
to
IPv4 Internet
::/0 to 6to4 relay via 6to4 interface
2002:9D3C:1:1::/64 to local subnet
6to4 Host to IPv6 Host (Pt 3)
6to4 router
6to4 host A
2002:9D3C:1:1::1
IPv4 Internet
6to4 host/router B
IPv6 Internet
6to4 relay
Routes:
::/0 to IPv6 Internet via
LAN interface
2002::/16 to IPv4 Internet
IPv6 Header:
Destination Address:
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
Source Address: 2002:9D3C:1:1::1
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
IPv6 tunneled over IPv4
Native IPv6 traffic
Questions?
Teredo Overview

Why do we need Teredo?

6to4 relies on public IPv4 address and IPv6
router functionality in edge device

Teredo (IPv6 NAT Traversal)

Address assignment and automatic tunneling
technology for unicast traffic between
IPv6/IPv4 nodes located behind one or more
IPv4 NATs on the IPv4 Internet

Encapsulates IPv6 traffic as a UDP message

IPv4 and UDP headers

Automatically adjusts behavior based on the
type
of the local NAT

RFC 4380
Teredo Traffic

IPv4 header

UDP header

Port 3544

IPv6 header

IPv6 payload

Extension headers

Upper Layer Protocol Data Unit
IPv4 Header
IPv6 Header
Upper Layer Protocol
Data Unit
UDP
Header
20 bytes
8 bytes
40 bytes
n
bytes
Extension
Headers
Types of NATs

Cone NAT

Internal Addr/Port mapped to external Addr/Port

Any inbound traffic to external Addr/Port allowed

Inbound traffic needs port mapping

Restricted NATs

Internal Addr/Port mapped to external Addr/Port

Only known inbound traffic to external Addr/Port allowed

Inbound traffic needs source
-
specific port mapping

Symmetric NAT

Maps same internal Addr/Port to different external Addr/Ports

Teredo in Windows XP and Server 2003 can work over only
cone and restricted NATs

Teredo in Windows Server 2008 and Windows Vista can work if
one of the Teredo clients is behind a symmetric NAT
Teredo Components
Teredo server
Teredo relay
NAT
Teredo client
IPv6 over IPv4 traffic
IPv4 Internet
IPv6 Internet
IPv6 traffic
NAT
Teredo client
Teredo
host
-
specific relay
IPv6
-
only host
IPv6 or IPv6
over IPv4 traffic
Teredo Address Format

Teredo Prefix

2001:0::/32
by
IANA

Teredo Server IPv4 Address

Flags

Cone (C) flag (high
-
order bit
)

Other bits used to obscure Teredo address (Windows Vista)

Obscured
External Port

XOR of External Port with 0xFFFF

Prevents “smart” NATs from translating

Obscured External Address

XOR of External Address with 0xFFFFFFFF

Prevents “smart” NATs from translating
Teredo Prefix
Flags
Obscured
External Port
Obscured
External Address
Teredo Server
IPv4 Address
32 bits
32 bits
16 bits
16 bits
32 bits
Teredo Addressing Example
Teredo server
Teredo relay
Cone
NAT
Teredo
client A
IPv4 Internet
IPv6 Internet
Restricted
NAT
157.60.0.1, UDP port 4096
131.107.0.1,
UDP port 8192
2001::CE49:7601:E866:EFFF:62C3:FFFE
2001::CE49:7601:2CAD:DFFF:7C94:FFFE
206.73.118.1
Teredo
client B
Teredo address:
2001:0:
ServerAddr
:
Flags
:
ObscExtPort
:
ObscExtAddr
Teredo Routing
NAT
Teredo client
IPv4 Internet
Teredo server
IPv6 Internet
Teredo relay
NAT
Routes:
::/0 on
-
link via Teredo interface
Routes:
::/0 on
-
link via Teredo interface
Routes:
2001::/
32 on
-
link via Teredo interface
::/0 to IPv6 Internet via
LAN
interface
Routes:
2001::/
32 to Teredo relay
Teredo host
-
specific relay
Teredo
client
Obtaining a Teredo Address
IPv4 Internet
Teredo Server 2
NAT
Teredo
Client
Teredo Server 1
1.
Router Solicitation

IPv6 tunneled as an IPv4 UDP message

3.
Router Solicitation

4.
Router Advertisement (with same IPv4 source
to detect a restricted NAT)

5.
Router Solicitation

6.
Router Advertisement (used to detect a
symmetric NAT)

2.
Router Advertisement (with different IPv4
source to detect cone NAT)
Maintaining the NAT Mapping
NAT
Teredo client
IPv4 Internet
Teredo server
NAT
1. Bubble to Teredo server

IPv6 tunneled as an IPv4 UDP message
IPv4 Internet
Initial Communication Between Teredo
Clients in Different Sites (Cone NAT)
Cone
NAT
Teredo
Client A
Teredo Server 1
Cone
NAT
Teredo Client B
Teredo Server 2
1.
Data
packet to Teredo Client B

IPv6 tunneled as an IPv4 UDP message
IPv4 Internet
Initial Communication Between Teredo
Clients in Different Sites (Restricted NATs)
Restricted
NAT
Teredo
Client A
Teredo Server 1
Restricted
NAT
Teredo Client B
Teredo Server 2
1. Bubble to Teredo Client B

IPv6 tunneled as an IPv4 UDP message

5. Data packet to Teredo Client B

4. Bubble to Teredo Client A

3. Forwarded bubble to Teredo Client B

2. Bubble to Teredo Server 2
IPv4 Internet
IPv6 Internet
Initial Communication from a Teredo Client
to an IPv6
-
only Host (Cone NAT)
Cone
NAT
Teredo
Relay
Teredo Client A
Teredo Server
IPv6
-
only Host
1. Echo Request to IPv6
-
only Host

IPv6 tunneled as an IPv4 UDP message
Native IPv6 traffic

2. Forwarded Echo Request to IPv6
-
only Host

3. Echo Reply to Teredo Client A

4. Forwarded Echo Reply to Teredo Client A

5.
Data packet
to IPv6
-
only Host

6. Forwarded packet to IPv6
-
only Host
IPv4 Internet
IPv6 Internet
Initial Communication from a Teredo Client
to an IPv6
-
only Host (Restricted NAT)
Restricted
NAT
Teredo
Relay
Teredo Client A
Teredo Server
IPv6
-
only Host
1. Echo Request to IPv6
-
only Host

IPv6 tunneled as an IPv4 UDP message
Native IPv6 traffic

2. Forwarded Echo Request to IPv6
-
only Host

3. Echo Reply to Teredo Client A

4. Bubble to Teredo Client A via Teredo Server

5. Bubble with origin indication to Teredo Client A

6. Bubble to Teredo Relay

7. Forwarded Echo Reply to Teredo Client A

8.
Data packet
to IPv6
-
only Host

9. Forwarded packet to IPv6
-
only Host
Questions?
IP
-
HTTPS
Overview

Why do we need
IP
-
HTTPS?

Node can be behind blocking firewall or proxy
server

Most firewalls and Web proxies forward
HTTPS
-
based traffic (TCP port 443)

IP
-
HTTPS

Address assignment and automatic tunneling
technology for unicast traffic
for
IPv6/IPv4
nodes located behind
firewalls or proxies
on
the IPv4 Internet

Encapsulates IPv6 traffic as
the payload of an
IPv4
-
based HTTPS stream

IPv4, TCP, and HTTP
headers
IP
-
HTTPS Packets

IPv6 packets from IP
-
HTTPS client

HTTP Request message

IPv6 packets from IP
-
HTTPS server

HTTP Response message
IPv4
HTTP
IPv6
packet
TCP
Encrypted by SSL session
IP
-
HTTPS Connection
1.
Establish TCP connection to port 443 on
the IP
-
HTTPS server
2.
Complete HTTPS
-
based handshake and
establish SSL
-
based encryption
3.
Perform router discovery

Client sends Router Solicitation

Server sends Router Advertisement
4.
Begin sending IPv6 traffic as HTTP
Request/Response messages
IP
-
HTTPS
Addressing
IPv4 Internet
IP
-
HTTPS client
IPv6
-
capable intranet
IP
-
HTTPS server
Example
prefix: 3FFE:2900:D005:99::/
64
3FFE:2900:D005:99:35EA:21F1:6117:CA80
IP
-
HTTPS
Routing
IPv4 Internet
IP
-
HTTPS client
IPv6 intranet
IP
-
HTTPS server
Example prefix:
3FFE:2900:D005:99::/
64
Routes:
3FFE:2900:D005:99::/
64
on link
via
IP
-
HTTPS
interface
::/0 to
IP
-
HTTPS server
via
IP
-
HTTPS
interface
Routes:
3FFE:2900:D005:99::/
64
on link
via
IP
-
HTTPS
interface
::/0 to
IPv6 intranet
via LAN
interface
Routes:
3FFE:2900:D005:99::/
64 to
IP
-
HTTPS server
IPv4 Internet
IP
-
HTTPS client
IPv6
-
capable intranet
IP
-
HTTPS server
Example prefix:
3FFE:2900:D005:99::/
64
3FFE:2900:D005:99:35EA:21F1:6117:CA80
3FFE:2900:D005:7C:2AA:FF:FE9A:21AC
IPv4 Header:
Destination Address:
131.107.27.121
Source Address:
154.60.0.1
IPv6 Header:
Destination Address:
3FFE:2900:D005:7C:2AA:FF:FE9A:21AC
Source Address:
3FFE:2900:D005:99:35EA:21F1:6117:CA80
IP
-
HTTPS Client
to
Intranet
Host
(Pt 1)
IPv6 tunneled over IPv4
Native IPv6 traffic
Routes:
3FFE:2900:D005:99::/
64
on link
via
IP
-
HTTPS
interface
::/0 to
IP
-
HTTPS server
via
IP
-
HTTPS
interface
IPv4 Internet
IP
-
HTTPS client
IPv6
-
capable intranet
IP
-
HTTPS server
Example prefix:
3FFE:2900:D005:99::/
64
3FFE:2900:D005:99:35EA:21F1:6117:CA80
3FFE:2900:D005:7C:2AA:FF:FE9A:21AC
IP
-
HTTPS Client
to
Intranet
Host
(Pt
2)
IPv6 Header:
Destination Address:
3FFE:2900:D005:7C:2AA:FF:FE9A:21AC
Source Address:
3FFE:2900:D005:99:35EA:21F1:6117:CA80
IPv6 tunneled over IPv4
Native IPv6 traffic
Routes:
3FFE:2900:D005:99::/
64
on link
via
IP
-
HTTPS
interface
::/0 to
IPv6 intranet
via LAN
interface
Questions?
IPv6 Transition Technologies

Transition Terms and Concepts

ISATAP

6to4

Teredo

IP
-
HTTPS
IPv6 Resources

Microsoft IPv6 Web site

http://
www.microsoft.com/ipv6

Understanding IPv6, 2
nd
Edition
Appendix A

IPv6
Addressing Review
The IPv6 Address Space

128
-
bit address space

2
128
possible addresses

340,282,366,920,938,463,463,374,607,431,768,211,
456 addresses (3.4 x 10
38
or 340 undecillion)

6.65 x 10
23
addresses for every square meter
of the Earth’s surface

128 bits to allow flexibility in creating a multi
-
level, hierarchical, routing infrastructure

64
-
bit subnet prefix and a 64
-
bit interface
identifier
IPv6 Address Syntax

IPv6 address in binary form
0010000000000001000011011011100000000000000000000010111100111011
0000001010101010000000001111111111111110001010001001110001011010

Divided along 16
-
bit boundaries
0010000000000001 0000110110111000 0000000000000000 0010111100111011
0000001010101010 0000000011111111 1111111000101000 1001110001011010

Each 16
-
bit block is converted to hexadecimal
and delimited with colons

2001:0DB8:0000:2F3B:02AA:00FF:FE28:9C5A

Suppress leading zeros within each block

2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
Compressing Zeros

A single contiguous sequence of 16
-
bit
blocks set to 0 can be compressed to “::”
(double
-
colon)

Example:

FE80:0:0:0:2AA:FF:FE9A:4CA2 becomes
FE80::2AA:FF:FE9A:4CA2

FF02:0:0:0:0:0:0:2 becomes FF02::2
IPv6 Prefixes

Express routes, address spaces, or address
ranges

IPv6 always uses
address
/
prefix
-
length
notation

Similar to CIDR notation

Examples

2001:DB8:0:2F3B::/64 for a subnet prefix

2001:DB8:3F::/48 for a route prefix

::/0 for the default route
IPv6 Address Summary

Link
-
local

Addresses begin with “FE80”

Unique for a single link

Unique local

Addresses begin with “
FC” or “FD”

Unique for
an organization

Global

Addresses begin with “2” or “3”

Unique for the entire IPv6 Internet

Multicast

Addresses begin with “FF