2009 Mar 07
| Direct Mapping | Dynamic mapping |
When a system starts sending data to another it starts by pushing the data down the TCP/IP stack with a destination IP address in the header. The IP layer on potential receivers aren't listening to the IP address because that is not the layer listening to the transmission medium. The data has to go through the physical network interface which implements the Data Layer and Physical Layers.
Each network physical interface unit is identified by the Medium Access Control (MAC) Address. To ensure these MAC addresses are globally unique each manufacturer is allocated a set of addresses, a different one of which is wired or hard coded into every Ethernet interface. This unique MAC address can be used to distinguisg every network interface.
When a system is about to send data to another system via a LAN, it must first determine the specific Data Link and Physical layers. The Addreess Resolution Protocol (ARP) is used to make this mapping or correspondence netween IP address and MAC address. This can apply to non Ethernet technologies, but RFC 826, is the specification for Ethernet.
The sending system must first send an ARP query which broadcats to all systems on the LAN. This query asks: "Who has the MAC addressed associated with IP address w.x.y.z", the destination IP. Every system receives this ARP query, and one is expected to give the ASP response, which says: "I have that IP address, and its MAC address is aa:bb:cc:dd:ee:ff".
The sending system, after receiving this response, will transmit its packet to the specified destination MAC address; and will store the IP to MAC translation in its ARP cache. The ARP cache and its entries are valid for a few minutes up to about a half hour. This serves to minimize ARP traffic on the LAN. ARP queries and responses do cross LAN bounderies, routers ignore them.
The mapping from IP address to physical address must take place at every step of the journey from the originator to the intended system. There are two types of physical addresses:
Address resolution for the first type (e.g. proNET) is easier and more and difficult for the latter (e.g. Ethernet).
2005-2009