- Key to achieving scalability:
- Avoid the intruding on non-dstination networks
- Why
connecting
multiple Ethernet LANs is
not scalable:
- When the location of
the destination is
unknown, a
Ethernet switch will
broadcast
the frame on
all attached networks
- When the location of
the destination is
unknown, a
Ethernet switch will
broadcast
the frame on
all attached networks
- Avoid having to
use broadcasting:
- Each node must maintain the location information of every node (on the entire Internet !!!
- Note:
- The amount of
location information is
too much for
any single node to
store.
- The information will be stored in a distributed manner to reduce storage size !!!
- The amount of
location information is
too much for
any single node to
store.
- Routing Table:
- Routing table =
contains
location information
on every IP destination
- How to
store routing table
in a
distributed manner:
-
Most router
only store
location information of
their "close-by" IP destinations
- Most router has
a default router
(that store a
larger routing table)
- If a destination is unknown (= not found in routing table), the router will send the IP packet to its default router for processing
-
Most router
only store
location information of
their "close-by" IP destinations
- Routing table =
contains
location information
on every IP destination
- (Simplified) structure of
the routing table:
- The routing table
contains
entries of the form:
Destination | Output port --------------+--------------- IP-addr1 | port1 IP-addr2 | port2 ... | ...(We will discuss the complete structure later)
(The routing table actually store IP network addresses.)
- The routing table
contains
entries of the form:
- Example:
Explanation:
- If router receives an IP packet destined for node IP1, it will send the packet out on its port X
- If router receives an IP packet destined for node IP5, it will send the packet out on its port Z
- If router receives an IP packet destined for node IP9, it will send the packet out on its port Y
- Command to
view the
routing table
in some computer:
netstat -r (r = routing table) Destination Gateway Genmask Flags MSS Window irtt Iface 170.140.150.0 * 255.255.254.0 U 0 0 0 eth0 link-local * 255.255.0.0 U 0 0 0 eth0 default defaultrouter.m 0.0.0.0 UG 0 0 0 eth0You can view the routing table in numerical format with:
netstat -r -n Destination Gateway Genmask Flags MSS Window irtt Iface 170.140.150.0 0.0.0.0 255.255.254.0 U 0 0 0 eth0 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0 0.0.0.0 170.140.150.254 0.0.0.0 UG 0 0 0 eth0
- Recall:
how to
forward an
IP packet by
a router
Explanation:
- The router will
accept the
frame because the
(Ethernet) destination is
its
network address
- The router will first
extract the
data portion
from the Ethernet frame
- The router then
examine the
destination IP address
in the
IP packet
(which is in the data portion)
- The router finds
the destination in its
routing table
- Finally, the
router uses the
appropriate port to
transmit the frame
(in the network format of
that port)
- The network interface card knows the proper frame format
- The router will
accept the
frame because the
(Ethernet) destination is
its
network address