Network traffic can be divided among three main classes: broadcast, multicast,and unicast. Each classification has a distinct characteristic that determineshow packets in that class are handled by networking hardware.
Broadcast Traffic
A broadcast packet is one that is sent to all ports on a network segment, regardlessof whether that port is a hub or switch.
All broadcast traffic is not created equally, however. There are layer 2and layer 3 forms of broadcast traffic. For instance, on layer 2, the MACaddress FF:FF:FF:FF:FF:FF is the reserved broadcast address, and any trafficsent to this address is broadcast to the entire network segment. Layer 3 alsohas a specific broadcast address.
The highest possible IP address in an IP network range is reservedfor use as the broadcast address. For example, in a network configuredwith a 192.168.0.xxx IP range and a 255.255.255.0 subnet mask, the address192.168.0.255 is the broadcast address.
In larger networks with multiple hubs or switches connected via differentmedia, broadcast packets transmitted from one switch reach all the way tothe ports on the other switches on the network, as they are repeated fromswitch to switch. The extent to which broadcast packets travel is called thebroadcast domain, which is the network segment where any computer can directlytransmit to another computer without going through a router. Figure 1-11
Figure 1-11: A broadcast domain extends to everything behind the current routed segment.
Our earlier example describing how routing relates to a neighborhoodalso provides good insight into how broadcast domains work. You can thinkof a broadcast domain as being like a neighborhood street. If you stand onyour front porch and yell, only the people on your street will be able to hearyou. If you want to talk to someone on a different street, you need to find away to speak to that person directly, rather than broadcasting (yelling) fromyour front porch.
Multicast Traffic
Multicast is a means of transmitting a packet from a single source to multipledestinations simultaneously. The goal of multicasting is to simplify this processby using as little bandwidth as possible. The optimization of this trafficlies in the number of times a stream of data is replicated in order to get to itsdestination. The exact handling of multicast traffic is highly dependent on itsimplementation in individual protocols.
The primary method of implementing multicast is via an addressingscheme that joins the packet recipients to a multicast group, which is howIP multicast works. This addressing scheme ensures that the packets cannotbe transmitted to computers to which they are not destined. In fact, IP devotesan entire range of addresses to multicast. If you see an IP address in the224.0.0.0 to 239.255.255.255 range, it is most likely multicast traffic.
Unicast Traffic
A unicast packet is transmitted from one computer directly to another. Thedetails of how unicast functions depend on the protocol using it.
For example, consider a device that wishes to communicate with a webserver. This is a one-to-one connection, so this communication processwould begin with the client device transmitting a packet to only the webserver. This form of communication is an example of unicast traffic.