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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
ARP
The Address Resolution Protocol (ARP) determines the MAC address and IP address correspondence for
a network device.
A local computer will maintain an ARP cache that is a table of MAC addresses and the corresponding IP
addresses. Before a connection with another computer is made, the local computer first checks its ARP
cache to determine whether the remote computer has an entry. If it does, the local computer reads the
remote computer’s MAC address and writes it into the destination field of the packets to be sent.
If the remote computer does not have an ARP cache entry, the local computer must send an ARP
request and wait for a reply.
When the local computer receives the ARP reply packet, the local ARP reads the IP MAC address pair,
and then checks the ARP cache for this entry. If there is an entry, it is updated with the new
information. If there is no entry, a new entry is made.
There are two possible cases when an ARP packet is received by a local computer. First, the local
computer is the target of the request. If it is, the local ARP replies by sending its MAC IP address pair
back to the requesting system. Second, if the local computer is not the target of the request, the packet
is dropped.
Multicasting
Multicasting is a group of protocols and tools that enable a single source point to send packets to
groups of multiple destination points with persistent connections that last for some amount of time.
The main advantage to multicasting is a decrease in the network load compared to broadcasting.
Multicast Groups
Class D IP addresses are assigned to a group of network devices that comprise a multicast group. The
four most significant four bits of a Class D address are set to “1110”. The following 28 bits is referred
to as the ‘multicast group ID’. Some of the range of Class D addresses are registered with the Internet
Assigned Numbers Authority (IANA) for special purposes. For example, the block of multicast addresses
ranging from 224.0.0.1 to 224.0.0.225 is reserved for use by routing protocols and some other low-level
topology discovery and maintenance protocols.
Figure 5-17. Class D Multicast Address
Some of the reserved IP multicast addresses are as follows:
Address Assignment
224.0.0.0
Base Address (reserved)
224.0.0.1
All Systems on this subnet
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