Arp; Multicasting - D-Link DGS-3308 Series User Manual

8-port Gigabit Ethernet Switch User's Guide
The software in a traditional router (or the hardware in a layer 3 switch) is designed to forward packets from one network
to another.
Routing is based upon the Network Address of the destination IP address. Each network device or computer has at least
one gateway address (the default gateway) and this is generally the best way to send packets out of the local network and
into the WAN or the Internet.
The network device or computer does not have to know the gateway address of the gateway to the destination network, only
the local gateway out of the local network. When there is no specific gateway address to send the packet to, it is sent to the
default gateway.
The gateway itself must know a lot more about which routes are available and where they go. To do this, a gateway
develops a routing table using a routing protocol that is designed to help gateways find each other and exchange
information about their local routes to other gateways.

ARP

The Address Resolution Protocol (ARP) is used to determine the association of IP addresses and MAC addresses for
network devices and computers.
When a packet is to be transmitted, the destination's IP address is first resolved into a MAC address. Network devices
and computers (that operate on layer 3) maintain a local ARP cache. This is a local database of IP addresses and
corresponding MAC addresses. If the destination IP address has an entry in the local ARP cache, the MAC address is
written to the packet's destination field and the packet is sent.
If the destination IP address does not have an entry in the local ARP cache, an ARP request must be sent to resolve the IP
address into a MAC address. The packet must wait for a response from the destination before being sent.
ARP requests that are received from the network have their IP and MAC address pair extracted. The local ARP cache is
then checked to see if there is already an entry for this pair. If an entry does not exist, the pair are added to the local ARP
cache. If the entry already exists, the ARP request is dropped.
If an ARP request is received and the receiving network device has the IP address the ARP request is trying to resolve, the
receiving device replies by giving its MAC address.

Multicasting

Multicasting allows a single network device to transmit packets to multiple destinations. These connections typically are
used over a considerable period of time. End nodes that are receiving the multicast transmissions are referred to as a
multicast group. Multicast group members can reside on different subnets and even on different networks.
Multicast Addressing
A multicast group is given an IP address where the first four bits of the address are set to '1110'. This is a Class D IP
address. The 28 bit number following the '1110' are referred to as the multicast group ID.
Some Class D addresses are reserved for special purposes. Addresses from 224.0.0.1 to 224.0.0.255 is reserved for use by
routing protocols and some low-level protocols. Addresses from 239.0.0.0 to 239.255.255.255 are used by administrative
applications for local networks.
The Class D IP address format is shown below:
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