Pilz PSSnet SHL 8T MRP - Manual (Page 30)

Entering the IP Parameters
PSSnet SHL - Basic Configuration
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2.1 IP Parameter Basics
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Lorenzo receives the letter and removes the outer envelope. From the in-
ner 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 table) for Juliet's MAC address. He writes her MAC address on
the outer envelope 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 SO/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 en-
velope.
In the MIB, Juliet finds Lorenzo listed under the variable hmNetGateway-
IPAddr as a means of communicating with Romeo. She therefore puts the
envelope with the IP addresses in a further envelope with Lorenzo's MAC
destination address.
The letter now travels back to Romeo via Lorenzo, the same way the first
letter traveled from Romeo to Juliet.
2.1.3 Classless Inter-Domain Routing
Class C with a maximum of 254 addresses was too small, and class B with
a maximum of 65534 addresses was too large for most users, as they would
never require so many addresses. This resulted in ineffective usage of the
class B addresses available.
Class D contains reserved multicast addresses. Class E is reserved for
experimental purposes. A gateway not participating in these experiments
ignores datagrams with these destination addresses.