Hirschmann MS30, Power MICE, MACH 4000, OCTOPUS, User Manual

Page: 31 / 196

Entering the IP parameters
Layer 2 Enhanced with Layer 2 Professional
Release 1.0 10/05
2.1 Basics IP parameter
31
Lorenzo receives the letter and removes the outer envelope. From the inner
envelope he recognizes that the letter is meant for Juliet. He places the inner
envelope in a new outer envelope and searches his address list (the ARP ta-
ble) for Juliet's MAC address. He writes her MAC address on the outer enve-
lope as the destination address and his own MAC address as the source
address. He then places the entire data packet in the mail box.
Juliet receives the letter and removes the outer envelope. She finds the inner
envelope with Romeo's IP address. Opening the inner envelope and reading
its contents corresponds to transferring the message to the higher protocol
layers of the ISO/OSI layer model.
Juliet would now like to send a reply to Romeo. She places her reply in an
envelope with Romeo's IP address as destination and her own IP address as
source. But where is she to send the answer? For she did not receive
Romeo's MAC address. It was lost when Lorenzo replaced the outer enve-
lope.
In the MIB, Juliet finds Lorenzo listed under the variable
hmNetGatewayI-
PAddr
as a means of communicating with Romeo. She therefore puts the en-
velope with the IP addresses in a further envelope with Lorenzo's MAC
destination address.
The letter then travels back to Romeo via Lorenzo, the same way the first let-
ter traveled from Romeo to Juliet.