Understanding the network.pdf - Back to Home

IP datagrams for which the local host does not have a route entry in its local routing
table.
Dynamically generated routing tables are usually found on ISs/routers rather than
end-stations. Dynamic routing tables are created by using a dynamic routing
protocol, and are required when multiple routers and network exit points exist in the
network. A dynamic routing protocol enables the router to build the routing table on
its own by communicating with other routers designated to share routing
information with each other. The routers then have conversations with each other
about the state of their links, their type and speed, how long they have been up, and
so on. Each router can use the information it has about the state of the network to
choose the best route path. TCP/IP dynamic routing protocols will be covered in
Chapter 8, "TCP/IP Dynamic Routing Protocols."
Regardless of how the routing table is created, it is imperative that the information
in the routing table be correct. If a host's or router's routing table has incorrect
information, it will not be able to forward datagrams properly and they will not be
delivered.
The actual route table lookup is handled by a process known as a match lookup. The
match lookup compares the datagram's network destination address to those listed
in the routing table. This lookup is performed by comparing the binary network
address to each network address that is in the routing table. The destination
network address is compared, reading left to right, until the longest match is found.
Look at the two following examples.
In Example 1, the router has two interfaces: e1:192.168.3.21 /24 and
s1:147.28.0.1 /30. The destination address of the datagram is 12.14.180.220. The
binary routing table looks like this:
Decimal Network Binary Network Address
Address
e1: 192.168.3.0 11000000.10101000.00000011.00000000
s1: 147.28.0.0 10010011.00011100.00000000.00000000
The binary address of the destination address looks like this:
Decimal Network Binary Network Address
Address
12.14.180.220 00001100.00001110.10110100.11011100