Because a network and communications device can have multiple addresses with each serving a specific requirement, the need for Address Mapping for issue resolution is essential to the smooth running of all network environments.
Hubs, bridges, Network Interface Cards (NIC), switches that use transparent bridging (workgroup switches for example) and layer two managed switches (LAN switches) all use layer two addresses (MAC/hardware Addresses) to perform their magic.
On the other, hand the vast majority of all other devices including routers and PCs use layer three logical addresses (such as IP Addresses) to perform their brand of magic. Because of this, the need to be able to map IP addresses (logical) to MAC addresses becomes imperative.
The Network Layer (Layer three of the OSI Reference Model) identifies the destination node’s logical network address (IP address) but must use MAC Addresses (OSI Layer two the Data Link Layer) to move the data over the network.
The solution to overcoming this paradox was to develop special protocols that have the ability to map the logical IP Addresses to the appropriate correct physical MAC Address of the intended destination node. In fulfilling this need, each protocol suite has its own special methods (usually different) and “helper” protocols. Here are a few of them:
Address Resolution Protocol (ARP)
ARP is the most commonly used protocol for mapping IP Addresses to MAC Address. This means that we know the IP Address but the MAC Address is unknown. Thus, we need to discover (learn) the MAC Address somehow. This is the method used by TCP/IP.
Hello Protocol
Enables network devices to learn the MAC Addresses of other network devices by broadcasting a hello message at boot time. Examination of the Hello protocol packets enables devices to learn what other devices are on the network, as well as what their MAC Addresses are.
Embedded MAC Addresses
Embedding MAC Addresses into the network layer address means that by using special algorithms and protocols that apply specific rules that all parties/entities to the forthcoming communication/conversation need to agree upon in advance prior to commencement of the communication/conversation.
Algorithmic Derivation
MAC Addresses can also be derived using specific algorithms.
The following systems use both embedded MAC Addresses and algorithmic derivation to discover unknown MAC Addresses and map them to the known IP Address:
- XNS – Xerox Network Systems
- IPX – Novell Internetwork Packet Exchange
- DECnet Phase IV
Reverse Address Resolution Protocol (RARP)
Performs the reverse procedure of that performed by ARP. It takes a known MAC Address and maps it to an unknown IP Address.
Remember that Network/IP Addresses are OSI Layer 3 (Network) functionality, which means that they are Virtual/Logical Addresses. The relationship between a network address and any specific device is logical and not fixed.
Network/Logical Addresses Derivation Based On:
- Physical Network Characteristics – such as the particular network segment location on which the device is located.
- No Physical Basis – Groupings that have no physical basis include devices belonging to a part of an AppleTalk zone.
End Systems
Assuming that they only have one physical network connection end systems require one Network Layer Address (OSI Layer 3) for each network layer protocol supported.
Routers and other internetworking devices require one network layer address per physical network connection for each network layer protocol supported.
For Example: If a router has three interfaces each of which is running AppleTalk, TCP/IP, and OSI then it must have three network layer addresses for each interface. In this case, the router will have nine network layer addresses.
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