As you may notice, the Ethernet II format provides the Ethernet type information in 2 bytes while the 802.3 SNAP requires 8 bytes to provides this information. Consequently, most TCP/IP installations use the Ethernet II frame format. The 802.3 SNAP format is typically used only when strict 802.3 compatibility is required.
3.4 Ethernet Addresses
Every Ethernet adapter board has a unique 48 bit physical assigned by the manufacturer of the adapter board. The first 24 bits, assigned by IEEE, are unique to the hardware manufacturer. The manufacturer then assigns a unique number to every adapter board using the remaining 24 bits. When a network adapter recognizes a packet containing its address in the "destination" field, it processes the packet; otherwise the adapter ignores it. Special address conventions allow a message to be broadcast to all stations or "multicast" to a group of stations.
3.5 Requirement for Higher Level Protocols
Based on the information presented in this previous section, it might appear that Ethernet, by itself, is sufficient for local area network communications. It would seem that application software could simply insert the destination address and the application data in the Ethernet packet and send it. The receiving software would be presented with the packet by Ethernet and would remove and process the application data. However, there are several problems with this approach:
1)The application programming interface at the data link level is fairly complex.
2)There are no widely available standard application program interfaces such as Berkeley sockets at the data link level.
3)Since the Ethernet physical address is determined by factory hardware settings, changing out a 2572 or a PC Network Interface Card will change the physical address of a PLC or a PC. Without a higher level protocol like TCP/IP, maintaining address data would be up to the application program.
4)The application would be local network dependent; the data could not be routed across different network types.
As a result, additional upper layer protocols such as TCP/IP are required to simplify programming, to provide additional services, and to provide a level of abstraction from the physical network. TCP/IP and upper layer protocols are discussed in the next section.
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