Broadcast transmission refers to a device sending a message to all the devices on a network in one-to-all communications.
A broadcast packet has a destination IP address with all ones (1s) in the host portion, or 32 one (1) bits.
Note
IPv4 uses broadcast packets. However, there are no broadcast packets with IPv6.
A broadcast packet must be processed by all devices in the same broadcast domain. A broadcast domain identifies all hosts on the same network segment. A broadcast may be directed or limited. A directed broadcast is sent to all hosts on a specific network. For example, in Figure 9-2 a host on the 172.16.4.0/24 network sends a packet to 172.16.4.255. Directed broadcasts are not very common in today’s networks. A limited broadcast is sent to 255.255.255.255. By default, routers do not forward broadcasts.
Figure 9-2 Broadcast Transmission
Broadcast packets use resources on the network and make every receiving host on the network process the packet. Therefore, broadcast traffic should be limited so that it does not adversely affect the performance of the network or devices. Because routers separate broadcast domains, subdividing networks can improve network performance by eliminating excessive broadcast traffic.
Multicast transmission reduces traffic by allowing a host to send a single packet to a selected set of hosts that subscribe to a multicast group.
A multicast packet is a packet with a destination IP address that is a multicast address. IPv4 has reserved the 224.0.0.0 to 239.255.255.255 addresses as a multicast range.
Hosts that receive particular multicast packets are called multicast clients. The multicast clients use services requested by a client program to subscribe to the multicast group.
Each multicast group is represented by a single IPv4 multicast destination address. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address, and packets addressed to its uniquely allocated unicast address.
Routing protocols such as OSPF use multicast transmissions, as shown in Figure 9-3. For example, routers enabled with OSPF communicate with each other using the reserved OSPF multicast address 224.0.0.5. Only devices enabled with OSPF will process these packets with 224.0.0.5 as the destination IPv4 address. All other devices will ignore these packets.
Figure 9-3 Multicast Transmission
This section discusses the different types of IPv4 addresses including public, private and legacy classful addressing.
Public and Private IPv4 Addresses (9.2.1)
Just as there are different ways to transmit an IPv4 packet, there are also different types of IPv4 addresses. Some IPv4 addresses cannot be used to go out to the internet, and others are specifically allocated for routing to the internet. Some are used to verify a connection and others are self-assigned. As a network administrator, you will eventually become very familiar with the types of IPv4 addresses, but for now, you should at least know what they are and when to use them.
Public IPv4 addresses are addresses which are globally routed between internet service provider (ISP) routers. However, not all available IPv4 addresses can be used on the internet. There are blocks of addresses called private addresses that are used by most organizations to assign IPv4 addresses to internal hosts.
In the mid-1990s, with the introduction of the World Wide Web (WWW), the private IPv4 addresses shown in Table 9-1 were introduced because of the depletion of IPv4 address space. Private IPv4 addresses are not unique and can be used internally within any network.
Note: The long-term solution to IPv4 address depletion was IPv6.
Table 9-1 The Private Address Blocks
Note
Private addresses are defined in RFC 1918 and sometimes referred to as RFC 1918 address space.