Tag: CCNA

To configure IPv6 in IOS, as with IPv4, addresses need to be assigned to interfaces. You can assign a single link-local address and multiple global addresses. In comparison to IPv4, IPv6 unicast routing is disabled by default and needs to be globally enabled.

First, we will use a command that automatically generates link-local address for the interface.

R01
R01(config)#interface GigabitEthernet 3
R01(config-if)#ipv6 enable

Link-Local Address Configuration

Let’s check what IPv6 address has been allocated by IOS.

R01
R01#show ipv6 interface
GigabitEthernet3 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::20C:29FF:FEB8:6296 
  No Virtual link-local address(es):
  No global unicast address is configured
  Joined group address(es):
    FF02::1
    FF02::1:FFB8:6296

As the listing shows, the link-local address has been automatically assigned. It is derived from the MAC address of the interface displayed in the listing below.

R01
R01#show interface GigabitEthernet3
GigabitEthernet3 is up, line protocol is up 
  Hardware is CSR vNIC, address is 000c.29b8.6296 (bia 000c.29b8.6296)

The interface has automatically joined 2 multicast groups – FF02::1, which is the all-nodes address, and solicited-node for this address -FF002::1:FFB8:6296.

See the details on different addresses format and how they are derived in this blog post.

Let’s manually assign the link-local address, so it is not modified EUI-64 based. “ipv6 enable” command can be removed in this case, as its purpose is to just allocate a link-local address. For link-local addresses, no prefix-length needs to be specified, as it has a fixed format.

R01
R01(config)#interface GigabitEthernet 3
R01(config-if)#ipv6 address FE80::1 link-local

Let’s check how the output has changed.

R01
R01#show ipv6 interface
GigabitEthernet3 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::1 
  No Virtual link-local address(es):
  No global unicast address is configured
  Joined group address(es):
    FF02::1
    FF02::1:FF00:1

Notice the new link-local address and solicited-node multicast group addresses.

The next example demonstrates that only one single link-local address is allowed. If a new address is typed in it will overwrite the previous one.

R01
R01(config)#interface GigabitEthernet 3
R01(config-if)#ipv6 address FE80::1 link-local
R01(config-if)#end
R01#show running-config interface Gi3
interface GigabitEthernet3
 no ip address
 ipv6 address FE80::1 link-local
R01(config)#interface GigabitEthernet 3
R01(config-if)#ipv6 address FE80::2 link-local
R01(config-if)#end
R01#show running-config interface Gi3
interface GigabitEthernet3
 no ip address
 ipv6 address FE80::2 link-local

IOS will also prevent typing in an address in the link-local prefix range without a link-local keyword.

R01
R01(config-if)#ipv6 address FE80::1/64
% Link local requires link-local keyword

Addresses in any other than link-local range cannot be used with the keyword.

R01
R01(config-if)#ipv6 address 2001::2 link-local 
% Invalid link-local address

Global Unicast Address Configuration

IOS automatically assigns link-local addresses as soon as you configure IPv6 address on the interface. In most cases, you will start your configuration with allocating addresses from global unicast or unique local ranges.

Diagram 2 shows lab topology that we will be using in the next examples.

First let’s configure R1’s interface and verify it’s settings. 

R01
R01(config)#interface GigabitEthernet 2.123
R01(config-subif)#encapsulation dot1Q 123
R01(config-subif)#ipv6 address 2001:0:0:123::1/64
R01(config-subif)#end
R01#show ipv6 interface Gi2.123
GigabitEthernet2.123 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::20C:29FF:FEB8:628C 
  No Virtual link-local address(es):
  Global unicast address(es):
    2001:0:0:123::1, subnet is 2001:0:0:123::/64 
  Joined group address(es):
    FF02::1
    FF02::1:FF00:1
    FF02::1:FFB8:628C

IOS automatically assigned a link-local address. The global unicast address is now assigned and the interface has joined the corresponding solicited-node multicast group – FF02::1:FF00:1.

The similar configuration is now applied to the remaining routers.

R02
R02(config)#interface GigabitEthernet  2.123
R02(config-subif)#encapsulation dot1q 123
R02(config-subif)#ipv6 address 2001:0:0:123::2/64

R03
R03(config)#interface GigabitEthernet 2.123
R03(config-subif)#encapsulation dot1Q 123
R03(config-subif)#ipv6 address 2001:0:0:123::3/64

R03(config-subif)#interface GigabitEthernet 2.34
R03(config-subif)#encapsulation dot1q 34
R03(config-subif)#ipv6 address 2001:0:0:34::3/64

R04
R04(config)#interface GigabitEthernet 2.34
R04(config-subif)#encapsulation dot1q 34
R04(config-subif)#ipv6 address 2001:0:0:34::4/64

Let’s now test connectivity to confirm that we can reach routers on the same segment. As in IPv4, the ping command recognizes IPv6 address format.

R01
R01#ping 2001:0:0:123::3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:123::3, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)
R01#ping 2001:0:0:123::2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:123::2, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)

The connectivity will not work without enabling ipv6 unicast-routing globally, as it is disabled by default in the version of IOS used in this example. Future versions most likely will have it enabled by default.

R01 (R02, R03, R04)
R01(config)#ipv6 unicast-routing 

Now the same subnet reachability is working.

R01#ping 2001:0:0:123::2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:123::2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

R01#ping 2001:0:0:123::3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:123::3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
R01#

Similar to the “show ip arp” command that displays IP-to-ARP information, in the IPv6 world, there is a “show ipv6 neighbors” command.

R01
R01#show ipv6 neighbors 
IPv6 Address                              Age Link-layer Addr State Interface
2001:0:0:123::2                             0 000c.29ae.3524  REACH Gi2.123
2001:0:0:123::3                             0 000c.29fe.c0ba  REACH Gi2.123
FE80::20C:29FF:FEAE:3524                    1 000c.29ae.3524  STALE Gi2.123
FE80::20C:29FF:FEFE:C0BA                    1 000c.29fe.c0ba  STALE Gi2.123

IOS provides another command that can automatically derive the host portion (modified EUI-64) of IPv6 address based on its MAC address. For readability, I’ve assigned the host portion of the address to match the router name. Each interface in IPv6 can have multiple global unicast addresses, so let’s assign the second address to R01.

R01
R01(config)#interface Gi2.123
R01(config-subif)#ipv6 address 2001:0:0:123::/64 eui-64 
R01(config-subif)#end
R01#show running-config interface Gi2.123
interface GigabitEthernet2.123
 encapsulation dot1Q 123
 ipv6 address 2001:0:0:123::1/64
 ipv6 address 2001:0:0:123::/64 eui-64
end

R01#show ipv6 interface
GigabitEthernet2.123 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::20C:29FF:FEB8:628C 
  No Virtual link-local address(es):
  Global unicast address(es):
    2001:0:0:123::1, subnet is 2001:0:0:123::/64 
    2001::123:20C:29FF:FEB8:628C, subnet is 2001:0:0:123::/64 [EUI]
  Joined group address(es):
    FF02::1
    FF02::2
    FF02::1:FF00:1
    FF02::1:FFB8:628C

Notice that the router now has 2 IP addresses allocated. As both global addresses share the same last 24-bits, they are mapped to the same solicited-node multicast group – FF02::1:FFB8:628C.

Static Routes Configuration

To enable full reachability we will setup static IPv6 routes. R3 knows about both networks, as it has directly attached interfaces in 2001:0:0:123::/64 and 2001:0:0:34::/64. All other routers require a single static route to a remote subnet. First, let’s configure the static route for R1.

R01
R01(config)#ipv6 route 2001:0:0:34::/64 Gi2.123 2001:0:0:123::3

Notice that the route has interface and next-hop information. This type of static route is called a fully specified static route. The next-hop address must be directly attached to the interface. The interface can be omitted, in which case the router will do a lookup to identify the egress interface for the next-hop address. This is called a recursive route lookup. With recursive routes, next-hop is not required to be directly attached.

Let’s check the resulting routing table using the “show ipv6 route” command to confirm that the static route is now present.

R01
R01#show ipv6 route
IPv6 Routing Table - default - 5 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
       B - BGP, R - RIP, H - NHRP, I1 - ISIS L1
       I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP
       EX - EIGRP external, ND - ND Default, NDp - ND Prefix, DCE - Destination
       NDr - Redirect, RL - RPL, O - OSPF Intra, OI - OSPF Inter
       OE1 - OSPF ext 1, OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1
       ON2 - OSPF NSSA ext 2, la - LISP alt, lr - LISP site-registrations
       ld - LISP dyn-eid, lA - LISP away, a - Application
S   2001:0:0:34::/64 [1/0]
     via 2001:0:0:123::3, GigabitEthernet2.123
C   2001:0:0:123::/64 [0/0]
     via GigabitEthernet2.123, directly connected
L   2001:0:0:123::1/128 [0/0]
     via GigabitEthernet2.123, receive
L   2001::123:20C:29FF:FEB8:628C/128 [0/0]
     via GigabitEthernet2.123, receive
L   FF00::/8 [0/0]
     via Null0, receive

Reachability to remote subnet now works, as shown in the next listing. As R4 doesn’t have a static route for return traffic at this stage, it is still not reachable.

R01
R01#ping 2001:0:0:34::3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:34::3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/5 ms
R01#ping 2001:0:0:34::4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:34::4, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)

As the next step, we will configure all remaining routers. R4 has connectivity only via R3 and is called stub router, so it will have an only default route configured instead of a specific route.

R02
R02(config)#ipv6 route 2001:0:0:34::/64 Gi2.123 2001:0:0:123::3

R04
R04(config)#ipv6 route ::/0 2001:0:0:34::3

Verification Commands

Let’s test that we have now connectivity from R1 to R4. Ping and traceroute commands work in a similar way as in IPv4.

R01
R01#ping 2001:0:0:34::4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:34::4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

R01(config)#do traceroute 2001:0:0:34::4
Type escape sequence to abort.
Tracing the route to 2001:0:0:34::4

1 2001:0:0:123::3 1 msec 1 msec 1 msec
2 2001:0:0:34::4 1 msec 1 msec 1 msec

To see detailed debug-level information on neighbor discovery we will use the “debug ipv6 nd” command. All debug commands must be used with care in a production environment, as they can cause performance degradation and in some cases can overload the router’s CPU.

In the example below, R01 doesn’t have information about R02’s MAC address. Debug shows that R1 sends Neighbor Solicitation messages and in response gets Neighbor Advertisement message with Link-Local Address (LLA) of R02. Debug also shows that after the exchange of Global Unicast information completed routers exchange link-local IPv6 information.

R01
R01#show ipv6 neighbors 
IPv6 Address                              Age Link-layer Addr State Interface
2001:0:0:123::3                             1 000c.29fe.c0ba  STALE Gi2.123
FE80::20C:29FF:FEFE:C0BA                    1 000c.29fe.c0ba  STALE Gi2.123

R01#debug ipv6 nd 
  ICMP Neighbor Discovery events debugging is on
  ICMP ND HA events debugging is ON
R01#terminal monitor
R01#
R01#ping 2001:0:0:123::2 
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:123::2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/14 ms
R01#
*Nov 18 00:08:01.516: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) DELETE -> INCMP
*Nov 18 00:08:01.519: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) Sending NS
*Nov 18 00:08:01.519: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) Queued data for resolution
*Nov 18 00:08:01.524: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) Received NA from 2001:0:0:123::2
*Nov 18 00:08:01.524: ICMPv6-ND: Validating ND packet options: valid
*Nov 18 00:08:01.524: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) LLA 000c.29ae.3524
*Nov 18 00:08:01.524: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::2) INCMP -> REACH
*Nov 18 00:08:01.528: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::1) Received NS from 2001:0:0:123::2
*Nov 18 00:08:01.528: ICMPv6-ND: Validating ND packet options: valid
*Nov 18 00:08:01.528: ICMPv6-ND: (GigabitEthernet2.123,2001:0:0:123::1) Sending NA to 2001:0:0:123::2
*Nov 18 00:08:06.586: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEB8:628C) Received NS from FE80::20C:29FF:FEAE:3524
*Nov 18 00:08:06.586: ICMPv6-ND: Validating ND packet options: valid
*Nov 18 00:08:06.586: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) Glean
*Nov 18 00:08:06.586: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) LLA 000c.29ae.3524
*Nov 18 00:08:06.586: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) INCMP -> STALE
*Nov 18 00:08:06.587: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEB8:628C) Sending NA to FE80::20C:29FF:FEAE:3524
*Nov 18 00:08:06.588: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) STALE -> DELAY
*Nov 18 00:08:11.649: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) DELAY -> PROBE
*Nov 18 00:08:11.650: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) Sending NS
*Nov 18 00:08:11.651: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) Received NA from FE80::20C:29FF:FEAE:3524
*Nov 18 00:08:11.651: ICMPv6-ND: Packet contains no options
*Nov 18 00:08:11.652: ICMPv6-ND: Validating ND packet options: valid
*Nov 18 00:08:11.652: ICMPv6-ND: Packet contains no options
*Nov 18 00:08:11.652: ICMPv6-ND: (GigabitEthernet2.123,FE80::20C:29FF:FEAE:3524) PROBE -> REACH

R01#show ipv6 neighbors 
IPv6 Address                              Age Link-layer Addr State Interface
2001:0:0:123::2                             0 000c.29ae.3524  REACH Gi2.123
2001:0:0:123::3                             4 000c.29fe.c0ba  STALE Gi2.123
FE80::20C:29FF:FEAE:3524                    2 000c.29ae.3524  STALE Gi2.123
FE80::20C:29FF:FEFE:C0BA                    4 000c.29fe.c0ba  STALE Gi2.123

Troubleshooting Commands

The other debug command we can use for troubleshooting is the “debug ipv6 packets” command. In the production environment always use access-list based filters to limit the amount of output.

R01
R01#ping 2001:0:0:34::4 
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:0:0:34::4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms
R01#
*Nov 18 00:16:37.609: IPv6-Fwd: Destination lookup for 2001:0:0:34::4 : i/f=GigabitEthernet2.123, nexthop=2001:0:0:123::3
*Nov 18 00:16:37.610: IPv6-Fwd: SAS picked source 2001:0:0:123::1 for 2001:0:0:34::4 (GigabitEthernet2.123)
*Nov 18 00:16:37.610: IPv6-Fwd: nexthop 2001:0:0:123::3,
*Nov 18 00:16:37.610: IPV6: source 2001:0:0:123::1 (local)
*Nov 18 00:16:37.610:       dest 2001:0:0:34::4 (GigabitEthernet2.123)
*Nov 18 00:16:37.610:       traffic class 0, flow 0x0, len 100+0, prot 58, hops 64, originating
*Nov 18 00:16:37.610: IPv6-Fwd: Created tmp mtu cache entry for 2001:0:0:123::1 2001:0:0:34::4 00000000
*Nov 18 00:16:37.610: IPv6-Fwd: L3 injection feature enabled: skipping pak_encap
*Nov 18 00:16:37.611: IPv6-Fwd: Destination lookup for 2001:0:0:123::1 : Local, i/f=GigabitEthernet2.123, nexthop=2001:0:0:123::1
*Nov 18 00:16:37.611: IPV6: source 2001:0:0:34::4 (GigabitEthernet2.123)
*Nov 18 00:16:37.611:       dest 2001:0:0:123::1 (GigabitEthernet2.123)
*Nov 18 00:16:37.611:       traffic class 0, flow 0x0, len 100+18, prot 58, hops 63, forward to ulp

This command produces detailed information on the packet forwarding, including information on which source IPv6 address and the outgoing interface were chosen.

This blog post covers the following CCNA blueprint topics:

1.9 Compare IPv6 address types

• 1.9.a Global unicast
• 1.9.b Unique local
• 1.9.c Link local
• 1.9.d Anycast
• 1.9.e Multicast
• 1.9.f Modified EUI 64

The article content can be useful in preparation for other certification exams and IPv6 gradually becomes as important as IPv4.

Address Format

IPv6 addresses are 128-bit long. An address is divided into 8 groups each containing 4 hexadecimal digits. Groups are separated from each other by colons. Each group is 16-bit long and called hextet.

Hexadecimal digit is called a nibble (4-bit long, half-byte) and can be any number between 0 up to letter F. Figure 1 displays the difference between IPv4 and IPv6 address formats and terms.

IPv6 addresses are much longer in size than IPv4 addresses. Zeroes in the address can compress to make it more readable using the following rules:

• Leading zeroes within a hextet can be removed. For example, 00FC can be compressed to FC. Using this rule, 0000 can be compressed to 0, as it requires at least a single-digit to be preserved. The IPv6 address from Figure 1 can be compressed to FE80:0:0:0:C4A8:D1BF:7583:1166.
• Hextets of all zeroes can be replaced with double-colon once. The address from Figure 1 can be compressed as FE80::C4A8:D1BF:7583:1166. Only one group of zero-hextets can be replaced with double-colon.

IPv6 addresses use prefix notation similar to IPv4 CIDR (Classless Inter-Domain Routing) to identify subnet portion of the address. For example,

2001:C3A9:D1BF:7423:2345:112A:BCDE:F119/64 

is part of the subnet:

2001:C3A9:D1BF:7423:0:0:0:0/64

Address Types

There are 3 types of IPv6 addresses:

• Unicast addresses are allocated to a single device. Traffic sent to a unicast address is delivered only to this interface.
• An anycast address can be assigned to many devices at the same time. Traffic sent to an anycast address will be delivered to the closest device.
• Multicast. The address associated with multiple hosts and traffic sent to a multicast address delivered to many devices at the same time.

Unicast and anycast use the same IPv6 address space and multicast has its own dedicated prefix range.

In comparison to IPv4, there are no broadcast addresses in IPv6, which used to represent the delivery of traffic to all interfaces. Multicast delivery is now replacing this functionality.

Address Space

IPv6 address space is managed by Internet Assigned Numbers Authority (IANA). Current address space allocation is available via this URL.  The special-purpose address registry is available here.

Figure 2 demonstrates how address IPv6 address blocks are distributed proportionally to the full address space. Global Unicast address space (2000::/3) is the block that IANA uses for the current allocation and its size should provide enough addresses in the foreseeable future. For example, the entire IPv4 address space would be much smaller than a pixel in Figure 2.

Unicast Address Types

0000::/3

Addresses in 0000::/3 range contain several reserved addresses, such as an unspecified address, loopback, and IPv4-mapped addresses.

Unspecified address, 0:0:0:0:0:0:0:0 or :: means that the address is not present. It cannot be used as a destination address in any packets, however, it is used as a source when a host doesn’t have an address allocated.

Loopback address is allocated address of 0:0:0:0:0:0:0:1/128 or ::1/128. As in IPv4, this address represents the host itself. It cannot be assigned to any of the physical interfaces. As a result, it will not be seen outside of the host, and routers will not forward packets sent to this address.

IPv4-mapped IPv6 addresses are used in Dual-Stack (IPv4/IPv6) systems, so IPv6 application can recognize packet delivered to IPv4 address. For example, packet going to IPv4 address 1.2.3.4 will be mapped to IPv6 address of 0:0:0:0:0:FFFF:1.2.3.4 or ::FFFF:1.2.3.4.

Figure 3 displays the IPv4-mapped IPv6 address format. For all diagrams in this post, black dotted-line annotations mean that the value is static. Yellow dotted-line annotations mean that the value is variable.

IPv4-compatible addresses are another way of encoding IPv4 within IPv6 address. However, they are deprecated and IPv4-mapped addresses should be used instead.

2000::/3

Addresses in 2000::/3 range is what called Global Unicast Address space. The current allocation to registries is listed here.

The format of addresses in this range is shown in Figure 4. Global Unicast Address starts with binary 001 and the first digit can be either 2 or 3.

The first part of the address is variable-length Global Routing Prefix, which can be allocated to a site. It follows by variable-length Subnet ID for allocation within that site. The second half of the address (64-bits) is used by interface ID. Interface ID must be unique on the subnet and are derived from the hardware (MAC) address of the device. The resulting format is called modified EUI-64.

The process of deriving address from MAC address is displayed in Figure 5.

E000::/3

The last /3 block is e000::/3. It contains Unique-Local Addresses, Link-Local Unicast Addresses, and Multicast Addresses.

Unique Local Addresses can be used within a site or between multiple sites or organizations. They are not routable on the Internet. They have very similar features as RFC1918 private IPv4 addresses. What makes Unique Local different, is that they are designed to be unique most of the time. Part of the address named Global ID is a 40-bit pseudo-random number that ensures that there is a very small probability of having the same address range as the other party.

The allocated range for Unique Local Addresses is FC::/7. However, the 8th bit of the address is a flag. When it has a value of 1 it means that the address is locally defined. This is the only value currently defined, so all addresses will start with FD.

As displayed in Figure 6, Unique Local Address consists of 3 parts: Global ID, Subnet ID, and an Interface ID. A single Global ID prefix can accommodate more than 65000 subnets.

Link-Local IPv6 Unicast Addresses are designed to be used on a single layer-2 domain. They must not appear as the source or destination address for packets traversing routers. Link-Local Addresses are used for local traffic, such as automatic address configuration and neighbor discovery.

The format of the Link-Local IPv6 Unicast Address is shown in the Figure 7.

Multicast Addresses

The multicast address format is shown in Figure 8. The first 8 bits are always 1s (FF). Next 4 bits are reserved for flags followed by 4 bits representing group scope.

The flag bits (0RPT) are shown in Figure 9. The first bit (left-most or high-order) is always 0.

R-flag, if set to 1, means Rendezvous Point address is encoded within the group address.

P-flag means that it is unicast prefix-based allocation. When an entity receives Unicast Global address allocation, it gets delegated a corresponding multicast range as well. When P-Flag is set to 1, multicast address contains encoded unicast prefix of owner-organization.

T-flag stands for transient. If set to 1, the address is non-permanent. If set to 0, the address is well-known and has been permanently assigned by IANA.

Scope bits define how far multicast traffic can propagate. Table below shows some of the scopes defined:

Scope bits	Hex	Scope
0001	1	Interface-Local
0010	2	Link-Local
0100	4	Admin-Local
0101	5	Site-Local
1000	8	Organization-Local
1110	E	Global

There are several reserved multicast addresses. The next table shows some of these addresses.

Address	Description
FF02:0000:0000:0000:0000:0000:0000:0001	All Nodes (Link-Local)
FF02:0000:0000:0000:0000:0000:0000:0002	All Routers (Link-Local)
FF02:0000:0000:0000:0000:0001:FFXX:XXXX	Solicited-Node Address (XX:XXXX – last 24 bits of Unicast Address)
FF3S:0000:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX	Source-Specific Multicast (FF3S::/32). S – is scope, X – group address bits