Broadcast Messaging

Broadcast  Messaging .

    

When hosts are connected using either a hub or a switch, a single local network is created. Within the local network it is often necessary for one host to be able to send messages to all the other hosts at the same time. This can be done using a message known as a broadcast. Broadcasts are useful when a host needs to find information without knowing exactly what other host can supply it or when a host wants to provide information to all other hosts in the same network in a timely manner.

 A message can only contain one destination MAC address. So, how is it possible for a host to contact every other host on the local network without sending out a separate message to each individual MAC?

 To solve this problem, broadcast messages are sent to a unique MAC address that is recognized by all hosts. The broadcast MAC address is actually a 48-bit address made up of all ones. Because of their length, MAC addresses are usually represented in hexadecimal notation. The broadcast MAC address in hexadecimal notation is FFFF.FFFF.FFFF. Each F in the hexadecimal notation represents four ones in the binary address.

 When a host receives a message addressed to the broadcast address, it accepts and processes the message as though the message was addressed directly to it. When a host sends a broadcast message, hubs and switches forward the message to every connected host within the same local network. For this reason, a local network is also referred to as a broadcast domain.

If too many hosts are connected to the same broadcast domain, broadcast traffic can become excessive. The number of hosts and the amount of network traffic that can be supported on the local network is limited by the capabilities of the hubs and switches used to connect them. As the network grows and more hosts are added, network traffic, including broadcast traffic, increases. It is often necessary to divide one local network, or broadcast domain, into multiple networks to improve performance..

MAC and IP

On a local Ethernet network, a NIC only accepts a frame if the destination address is either the broadcast MAC address, or else corresponds to the MAC address of the NIC.

 Most network applications, however, rely on the logical destination IP address to identify the location of the servers and clients.

 What if a sending host only has the logical IP address of the destination host? How does the sending host determine what destination MAC address to place within the frame?

 The sending host can use an IP protocol called address resolution protocol (ARP) to discover the MAC address of any host on the same local network.

Address Resoulation Protocol (ARP)

ARP uses a three step process to discover and store the MAC address of a host on the local network when only the IP address of the host is known.

 1. The sending host creates and sends a frame addressed to a broadcast MAC address. Contained in the frame is a message with the IP address of the intended destination host.

 2. Each host on the network receives the broadcast frame and compares the IP address inside the message with its configured IP address. The host with the matching IP address sends its MAC address back to the original sending host.

 3. The sending host receives the message and stores the MAC address and IP address information in a table called an ARP table.

 Once the sending host has the MAC address of the destination host in its ARP table, it can send frames directly to the destination without doing an ARP request.

Distribution Layer

As networks grow, it is often necessary to divide one local network into multiple Access Layer networks. There are many ways to divide networks based on different criteria, including:

Physical location

Logical function

Security requirements

Application requirements

The Distribution Layer connects these independent local networks and controls the traffic flowing between them. It is responsible for ensuring that traffic between hosts on the local network stays local. Only traffic that is destined for other networks is passed on. The Distribution Layer can also filter incoming and outgoing traffic for security and traffic management.

Networking devices that make up the Distribution Layer are designed to interconnect networks, not individual hosts. Individual hosts are connected to the network via Access Layer devices, such as hubs and switches. The Access Layer devices are connected to each other via the Distribution Layer device, such as routers.