- Properties and
operation of a
bridge:
- A bridge has
promiscuous ports:
Promiscuous port:
- Promiscuous port = a network port that receives every (Ethernet) frame on the network
- A bridge makes its
forwarding decisions using the
physical (Ethernet) addresses
- A bridge will
repeat a
(received) frame
verbatim
- A bridge has
promiscuous ports:
- A router operates
as follows:
- A router has
selective (= non-promiscuous)
ports:
Selective port:
- Selective port = a network port that only receives an (Ethernet) frame that contains its own address as destination
- A router makes its
routing decision using
logical (IP) addresses
Note:
- The logical IP address is found in the data portion (IP packet) of the frame !!!
- The received frame and
the transmitted frame
(by the router) contain
different physical addresses:
Notice that:
- The physical addresses
are
different !!!
- But the logical addresses (in the IP packet) remains the same
- The physical addresses
are
different !!!
- A router has
selective (= non-promiscuous)
ports:
- Forwarding:
- Forwarding =
sending a message
to another network using
physical addresses
(e.g., Ethernet address)
Example:
- Forwarding
will not change
the frame
- In the above figure, you can see that the frame (A,I) is repeated verbatim onto a different Ethernet segment !!!
- Forwarding will therefore
only work with
homogeneous networks
- Because there is no protocol translation taking place !
- Forwarding =
sending a message
to another network using
physical addresses
(e.g., Ethernet address)
- Routing:
- Routing =
sending a message to
another network using
logical addresses
(e.g., IP address)
Example:
- Routing
will change the
frame:
- Before: the frame was
an Ethernet frame
- After: the frame is
a Token bus frame
- Before: the frame was
an Ethernet frame
- Routing will work even on heterogeneous networks !!!
- Routing =
sending a message to
another network using
logical addresses
(e.g., IP address)