Tiny Modbus/TCP to RTU/ASCII Gateway
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Appendix B: Glossary
1. ARP (Address Resolution Protocol)
The Address Resolution Protocol (ARP) is a telecommunication protocol that is used to convert an IP
address to a physical address, such as an Ethernet address.
Consider two machines A and B that share the same physical network. Each has an assigned IP
address IP
A
and IP
B
, and a MAC address, MAC
A
and MAC
B
. The goal is to devise a low-level software
application that hides the MAC addresses and allows higher-level programs to work only with the IP
addresses. Ultimately, however, communication must be carried out by the physical networks using
whatever MAC address scheme the hardware supplies.
Suppose machine A wants to send a packet to machine B across a physical network to which they
are both attached, but A only has the Internet address for B, IP
B
. The question arises: how does A
map that address to the MAC address for B, MAC
B
?
ARP provides a method of dynamically mapping 32-bit IP address to the corresponding 48-bit MAC
address. The term dynamic is used since the mapping is performed automatically and is normally
not a concern for either the application user or the system administrator.
2. Clients and Servers
The client-server paradigm uses the direction of initiation to categorize whether a program is a
client or server. In general, an application that initiates peer-to-peer communication is called a
client. End users usually invoke client programs when they use network services.
By comparison, a server is any program that waits for incoming requests from a client program. The
server receives a request from a client, performs the necessary action sand returns the result to the
client.
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