ModSnmp Manual v3.14 - 1.1. What is ModSnmp?
ModSnmp is an SNMP to Modbus bridge - that is, it converts between SNMP (Simple Network Management Protocol) and the Modbus protocol.
ModSnmp acts as an SNMP agent , handling requests from SNMP managers . It handles an SNMP request by sending a corresponding Modbus request (or requests) to a Modbus device (or devices)  and using their responses to construct an SNMP response. Note that ModSnmp acts as a Modbus client  when it communicates with Modbus devices.
In this configuration example, the SNMP manager (B) and ModSnmp (A) are on 2 independent machines, but they might equally well be on the same machine if the location of devices and their connections allow it. ModSnmp produces an MIB script file (F) containing the OIDs needed by the SNMP manager. This can be imported to the manager directly or by a 'converter' application (C). On the left side of the figure ModSnmp communicates with local Modbus devices over RS232 (Slave #1), RS485 (Slaves #2-5), and with more remote devices over the internet or an intranet (Slaves #6-11, #12-15).
In addition to being a SNMP-Modbus bridge, ModSnmp can also act as a Modbus/TCP server. This enables Modbus clients, as well as SNMP managers, to access the Modbus devices via ModSnmp.
 An SNMP agent may also be called an "SNMP command responder". In normal network terminology, it would be called an "SNMP server".
 An SNMP manager may also be called a "Network Management Station" (NMS) or an "SNMP command generator". In normal network terminology, it would be called a "SNMP client".
 A Modbus device is traditionally called a "Modbus slave". In normal network terminology, it would be called a "Modbus server".
 A Modbus client is traditionally called a "Modbus master".