Multicast IP Addresses

Last Updated on Thu, 01 Feb 2024 | Routing TCP IP

The IANA has set aside Class D IP addresses for use as multicast addresses According to the first octet rule, as described in Chapter 2, "TCP/IP Review," of Volume /, the first four bits of a Class D address are always 1110, as shown in Figure 5-4 Finding the minimum and maximum 32-bit numbers within this constraint, the range of Class D addresses is 224 0 0 0-239 255 255 255

Figure 5-4 Class D Addresses Are in the Range 224 0 0 0-239 255 255 255

Rule

Class A:

First bit is always 0

Minimums and maximums

00000000 = 0 01111111 = 127

Decimal range

* 0 and 127 are reserved

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Readers' Questions

  • patricia
    What is the functionality of a multicast ip address?
    8 months ago
    • A multicast IP address is a network addressing method used to send a single packet to a group of recipients on a computer network. It is an extension of the Internet Protocol and falls within the class D address range. Multicast IP addresses are used for applications where multiple recipients need to receive the same information simultaneously, such as streaming media applications and online gaming.

    Class B:

    10000000 =

    : 128

    128

    - 191

    First two bits are always 10

    10111111 =

    191

    Class C:

    11000000 =

    : 192

    192

    -223

    First three bits are always 110

    11011111 =

    223

    Class D:

    11100000 =

    224

    224

    -239

    First four bits are always 1110

    11101111 =

    239

    Unlike the Class A, B, and C address ranges, the Class D range is "flat"—that is, subnetting is not used, as demonstrated by Figure 5-5 Therefore, with 28 variable bits, 228 (more than 268 million) multicast groups can be addressed out of the Class D space

    Figure 5-5 Unlike Class A, B, and C IP Addresses, Class D Addresses Do Not Have a Network Portion and a Host Portion

    Class A: Class B:

    Class C: Class D:

    28-bit Group ID

    N

    H

    H

    H

    N

    N

    H

    H

    N

    N

    N

    H

    A multicast group is defined by its multicast IP address, groups may be permanent or transient Permanent refers to the fact that the group has a permanently assigned address, not that members are permanently assigned to the group In fact, hosts are free to join or leave any group Transient groups are, as you might guess, groups that do not have a permanent existence—like a videoconference group An unreserved address is assigned to the group and is relinquished when the group ceases to exist

    Table 5-1 shows some of the well-known addresses assigned to permanent groups by the IANA You have encountered most of these addresses before, when you studied the routing protocols to which they are assigned For example, you know that on a multiaccess network, OSPF DRothers send updates to the OSPF DR and BDR at 224 0 0 6; the DR sends packets to the DRothers at 224 0 0 5

    Table 5-1 Some Weil-Known Reserved Multicast Addresses

    Table 5-1 shows some of the well-known addresses assigned to permanent groups by the IANA You have encountered most of these addresses before, when you studied the routing protocols to which they are assigned For example, you know that on a multiaccess network, OSPF DRothers send updates to the OSPF DR and BDR at 224 0 0 6; the DR sends packets to the DRothers at 224 0 0 5

    Table 5-1 Some Weil-Known Reserved Multicast Addresses

    Address

    Group

    224 0 0 1

    All systems on this subnet

    224 0 0 2

    All routers on this subnet

    224 0 0 4

    DVMRP routers

    224 0 0 5

    All OSPF routers

    224 0 0 6

    OSPF designated routers

    224 0 0 9

    RIP-2 routers

    224 0 0 10

    EIGRP routers

    224 0 0 13

    PIM routers

    224 0 0 15

    CBT routers

    224 0 1 39

    Cisco-RP-Announce

    224 0 1 40

    Cisco-RP-Discovery

    The IANA reserves all the addresses in the range 224 0 0 0-224 255 255 255 for routing protocols and other network maintenance functions Multicast routers do not forward packets with a destination address from this range There are also addresses outside of this range that are reserved for open and commercial groups, for example, 224 0 1 1 is reserved for the Network Time Protocol (NTP), 224 0 1 8 is assigned to SUN NIS+, and 224 0 6 0-224 0 6 127 are assigned to the Cornell ISIS Project Yet another reserved range is 239 0 0 0-239 255 255 255 The use of this last group of addresses is discussed in the section "Multicast Scoping" later in this chapter For a complete list of reserved Class D addresses, see Appendix C, "Reserved Multicast Addresses," or RFC 1700

    A group member's network interface card (NIC) also must be multicast-aware When a host joins a group, the NIC determines a predictable MAC address To accomplish this, all multicast-aware Ethernet, Token Ring, and FDDI NICs use the reserved IEEE 802 address 0100 5E00 0000 to determine a unique multicast MAC It is significant that the eighth bit of this address is 1; that bit, in the 802 format, is the Individual/Group (I/G) bit When set, it indicates that the address is a multicast address

    Continue reading here: Internet Group Management Protocol IGMP

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