Multicast IP Addresses
Multicast applications consistently use a multicast IP address. This multicast abode represents the
multicast appliance and is referred to as a multicast group. Unlike a unicast IP address, which
uniquely identifies a distinct IP host, a multicast abode acclimated as a destination abode on an IP
packet signifies that the packet is accustomed cartage for a specific multicast application. For example,
if a multicast packet is traveling over a arrangement with a destination abode 225.5.5.5, it is
proclaiming to the arrangement accessories that, “I am accustomed cartage for the multicast appliance that
uses multicast accumulation abode 225.5.5.5; do you appetite it?” A multicast abode is never assigned to
a arrangement device, so it is never acclimated as a antecedent address. A antecedent abode on a multicast packet,
or any IP packet, is consistently a unicast address.
Multicast Abode Ambit and Structure
The Internet Assigned Numbers Authority (IANA) has assigned chic D IP addresses to multicast
applications. The aboriginal 4 $.25 of the aboriginal octet for a chic D abode are consistently 1110. IP multicast
addresses ambit from 224.0.0.0 through 239.255.255.255. As these addresses are acclimated to represent
multicast groups (applications) and not hosts, there is no charge for a subnet affectation for multicast
addresses because they are not hierarchical. In added words, there is alone one claim for a
multicast address: The aboriginal 4 $.25 of the aboriginal octet charge be 1110. The aftermost 28 $.25 are unstructured.
Well-Known Multicast Addresses
IANA controls the appointment of IP multicast addresses. To bottle multicast addresses, IANA
is afraid to accredit alone IP multicast addresses to new applications afterwards a good
technical justification. However, IANA has assigned alone IP multicast addresses to popular
network protocols.
Multicast IP Addresses 541
IANA has assigned several ranges of multicast IP addresses for specific types of reasons. Those
types are as follows:
■ Abiding multicast groups, in the ambit 224.0.0.0–224.0.1.255
■ Addresses acclimated with Source-Specific Multicast (SSM), in the ambit 232.0.0.0–
232.255.255.255
■ GLOP addressing, in the ambit 233.0.0.0–233.255.255.255
■ Clandestine multicast addresses, in the ambit 239.0.0.0–239.255.255.255
This area provides some insights into anniversary of these four types of aloof IP multicast
addresses. The blow of the multicast addresses are referred to as brief groups, which are
covered afterwards in this affiliate in the area “Multicast Addresses for Brief Groups.”
Multicast Addresses for Abiding Groups
IANA has aloof two ranges of abiding multicast IP addresses. The capital acumen between
these two ranges of addresses is that the aboriginal ambit is acclimated for packets that should not be forwarded
by routers, and the additional accumulation is acclimated back packets should be forwarded by routers.
The ambit of addresses acclimated for bounded (not routed) purposes is 224.0.0.0 through 224.0.0.255.
These addresses should be somewhat accustomed from the acquisition agreement discussions beforehand in the
book; for example, the 224.0.0.5 and 224.0.0.6 IP addresses acclimated by OSPF fit into this aboriginal range
of abiding addresses. Added examples accommodate the IP multicast destination abode of 224.0.0.1,
which specifies that all multicast-capable hosts on a bounded arrangement articulation should appraise this
packet. Similarly, the IP multicast destination abode of 224.0.0.2 on a packet specifies that all
multicast-capable routers on a bounded arrangement articulation should appraise this packet.
The ambit of abiding accumulation addresses acclimated back the packets should be baffled is 224.0.1.0
through 224.0.1.255. This ambit includes 224.0.1.39 and 224.0.1.40, which are acclimated by Ciscoproprietary
Auto-Rendezvous Point (Auto-RP) protocols (covered in Affiliate 17). Table 16-2
shows some of the acclaimed addresses from the abiding abode range.
Table 16-2 Some Well-Known Aloof Multicast Addresses
Address Usage
224.0.0.1 All multicast hosts
224.0.0.2 All multicast routers
224.0.0.4 DVMRP routers
224.0.0.5 All OSPF routers
224.0.0.6 OSPF appointed routers
542 Affiliate 16: Introduction to IP Multicasting
Multicast Addresses for Source-Specific Multicast Applications and Protocols
IANA has allocated the ambit 232.0.0.0 through 232.255.255.255 for SSM applications and
protocols. The purpose of these applications is to acquiesce a host to baddest a antecedent for the multicast
group. SSM makes multicast acquisition efficient, allows a host to baddest a better-quality source, and
helps arrangement administrators abbreviate multicast denial-of-service (DoS) attacks.
Multicast Addresses for GLOP Addressing
IANA has aloof the ambit 233.0.0.0 through 233.255.255.255 (RFC 2770), alleged GLOP
addressing, on an beginning basis. It can be acclimated by anyone who owns a registered autonomous
system cardinal (ASN) to actualize 256 all-around multicast addresses that can be endemic and acclimated by the
entity. IANA affluence addresses to ensure all-around character of addresses; for agnate reasons,
each free arrangement should be appliance an assigned altered ASN.
By appliance a amount of 233 for the aboriginal octet, and by appliance the ASN for the additional and third octets, a
single free arrangement can actualize globally altered multicast addresses as authentic in the GLOP
addressing RFC. For example, the free arrangement appliance registered ASN 5663 could covert
ASN 5663 to bifold (0001011000011111). The aboriginal 8 bits, 00010110, equals 22 in decimal
notation, and the aftermost 8 bits, 00011111, equals 31 in decimal notation. Mapping the aboriginal 8 $.25 to
the additional octet and the aftermost 8 $.25 to the third octet in the 233 ambit addresses, the article who
Address Usage
224.0.0.9 RIPv2 routers
224.0.0.10 EIGRP routers
224.0.0.13 PIM routers
224.0.0.22 IGMPv3
224.0.0.25 RGMP
224.0.1.39 Cisco-RP-Announce
224.0.1.40 Cisco-RP-Discovery
NOTE Alone IGMPv3-capable hosts can use the SSM feature. IGMPv3 is a new protocol. At
the time of this writing, a actual bound cardinal of IGMPv3 applications were available. Hence,
use of these addresses is minimal.
Table 16-2 Some Well-Known Aloof Multicast Addresses (Continued)
Multicast IP Addresses 543
owns the ASN 5663 is automatically allocated the abode ambit 233.22.31.0 through
233.22.31.255.
Multicast Addresses for Clandestine Multicast Domains
The aftermost of the aloof multicast abode ranges mentioned actuality is the ambit of administratively
scoped addresses. IANA has assigned the ambit 239.0.0.0 through 239.255.255.255 (RFC 2365)
for use in clandestine multicast domains, abundant like the IP unicast ranges authentic in RFC 1918, namely
10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. IANA will not accredit these administratively
scoped multicast addresses to any added agreement or application. Arrangement administrators are free
to use multicast addresses in this range; however, they charge configure their multicast routers to
ensure that multicast cartage in this abode ambit does not leave their multicast area boundaries.
Multicast Addresses for Brief Groups
When an action wants to use globally altered unicast addresses, it needs to get a block of
addresses from its ISP or from IANA. However, back an action wants to use a multicast
address for a all-around multicast application, it can use any multicast abode that is not allotment of the
well-known abiding multicast abode amplitude covered in the antecedent sections. These remaining
multicast addresses are alleged brief groups or brief multicast addresses. This agency that
the absolute Internet charge allotment the brief multicast addresses; they charge be dynamically allocated
when bare and charge be appear back no best in use.
Because these addresses are not assuredly assigned to any application, they are alleged transient.
Any action can use these multicast addresses afterwards acute any allotment or permission
from IANA, but the action is accepted to absolution these multicast addresses afterwards their use. At
the time of this writing, there is no accepted adjustment accessible for appliance the brief multicast
addresses. However, a abundant accord of assignment is actuality done by IETF to ascertain and apparatus a
standard adjustment for dynamically allocating multicast addresses.
Summary of Multicast Abode Ranges
Table 16-3 summarizes assorted multicast abode ranges and their use.
NOTE GLOP is not an acronym and does not angle for anything. One of the authors of RFC
2770, David Meyer, started apropos to this ambit of addresses as “GLOP” addressing, and since
then the ambit has been articular by the name GLOP addressing.
544 Affiliate 16: Introduction to IP Multicasting
Mapping IP Multicast Addresses to MAC Addresses
Assigning a Layer 3 multicast abode to a multicast accumulation (application) automatically generates
a Layer 2 multicast address. Figure 16-6 shows how a multicast MAC abode is affected from
a Layer 3 multicast address. The MAC abode is formed appliance an IEEE-registered OUI of
01005E, again a bifold 0, and again the aftermost 23 $.25 of the multicast IP address. The adjustment is
identical for Ethernet and Fiber Distributed Data Interface (FDDI).
Table 16-3 Multicast Abode Ranges and Their Use
Multicast Abode Ambit Usage
224.0.0.0 to 239.255.255.255 This ambit represents the absolute IPv4 multicast abode space. It
is aloof for multicast applications.
224.0.0.0 to 224.0.0.255 This ambit is allotment of the abiding groups. Addresses from this
range are assigned by IANA for arrangement protocols on a local
segment. Routers do not advanced packets with destination
addresses acclimated from this range.
224.0.1.0 to 224.0.1.255 This ambit is additionally allotment of the abiding groups. Addresses from
this ambit are assigned by IANA for the arrangement protocols that
are forwarded in the absolute network. Routers advanced packets
with destination addresses acclimated from this range.
232.0.0.0 to 232.255.255.255 This ambit is acclimated for SSM applications.
233.0.0.0 to 233.255.255.255 This ambit is alleged the GLOP addressing. It is acclimated for
automatically allocating 256 multicast addresses to any
enterprise that owns a registered ASN.
239.0.0.0 to 239.255.255.255 This ambit is acclimated for clandestine multicast domains. These
addresses are alleged administratively scoped addresses.
Remaining ranges of addresses
in the multicast abode space
Addresses from these ranges are alleged brief groups. Any
enterprise can admeasure a multicast abode from the transient
groups for a all-around multicast appliance and should absolution it
when the appliance is no best in use.
Multicast IP Addresses 545
Figure 16-6 Calculating a Multicast Destination MAC Abode from a Multicast Destination IP Address
To accept the mechanics of this process, use the afterward six steps, which are referenced by
number in Figure 16-6:
Step 1 Convert the IP abode to binary. Notice the aboriginal 4 bits; they are always
1110 for any multicast IP address.
Step 2 Replace the aboriginal 4 $.25 1110 of the IP abode with the 6 hexadecimal digits
(or 24 bits) 01-00-5E as multicast OUI, in the absolute amplitude of 12 hexadecimal
digits (or 48 bits) for a multicast MAC address.
Step 3 Replace the abutting 5 $.25 of the bifold IP abode with one bifold 0 in the
multicast MAC abode space.
Step 4 Copy the aftermost 23 $.25 of the bifold IP abode in the aftermost 23-bit amplitude of the
multicast MAC address.
Step 5 Convert the aftermost 24 $.25 of the multicast MAC abode from bifold to
6 hexadecimal digits.
Step 6 Combine the aboriginal 6 hexadecimal digits 01-00-5E with the aftermost 6 hexadecimal
digits, affected in Step 5, to anatomy a complete multicast MAC
address of 12 hexadecimal digits.
Unfortunately, this adjustment does not accommodate a altered multicast MAC abode for anniversary multicast
IP address, because alone the aftermost 23 $.25 of the IP abode are mapped to the MAC address. For
example, the IP abode 238.10.24.5 produces absolutely the aforementioned MAC address, 0x01-00-5E-0A-
18-05, as 228.10.24.5. In fact, because 5 $.25 from the IP abode are consistently mapped to 0, 25 (32)
- 0 0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1
228.10.24.5
01-00-5E-0A-18-05
Binary
1 1 1 0 0 1 0 0 . 0 0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1
Hex
01-00-5E
01-00-5E - 0 A - 1 8 - 0 5
Always
Hex
Multicast
IP Address
Multicast
MAC
Address
IP Address
In Binary
1
2
6
3
5
. . . . . . 4 . . . . . .
different chic D IP addresses aftermath absolutely the aforementioned MAC address. IETF credibility out that the
chances of two multicast applications on the aforementioned LAN bearing the aforementioned MAC abode are very
low. If it happens accidentally, a packet from a altered IP multicast appliance can be identified
at Layer 3 and discarded; however, arrangement administrators should be accurate back they implement
multicast applications so that they can abstain appliance IP addresses that aftermath identical MAC
addresses.