In this new internetwork, a single Class B IP network,
150.1.0.0, is used. This IP network is subdivided into three subnetsnamely,
150.1.1.0, 150.1.2.0, and 150.1.3.0. In this case, subnetting has been set up to
work like it did in Chapter 10, with
all hosts in the same IP subnet having the same value in the first 3 octets of
their IP addresses. (Notice that the IP addresses of the host computers, as well
as the routers, have changed, as compared to the earlier examples in this
chapter.)
Now take a closer look at the routing tables. In the earlier
example, the list of destinations was a list of Class A, B, and C network
numbers. In this case, the list shows the IP subnet numbers. To route packets
correctly, the router needs a routing table that lists all the possible groups
of IP addresses. As you read in Chapter
10, a subnet is just a subset of a larger class A, B, or C networkin other
words, just a smaller group of addresses.
Hannah, R1, and R2 use the same basic logic as before. When
Hannah sends a packet to the web server, the destination address is 150.1.3.3.
Hannah's default gateway still points to R1's Ethernet1 IP address. Hannah's
logic works like it did without subnettting, with Hannah sending an ARP
broadcast for IP address 150.1.1.250, R1 replying, and Hannah sending a frame to
R1 with the IP packet inside the Ethernet frame.
R1's logic is also the same as before. R1 receives the frame,
and if it is error free, it extracts the IP packet. The destination address is
150.1.3.3 in this case, so R1 looks at its IP routing table and finds an entry
for 150.1.3.0. R1 knows that this subnet includes addresses 150.1.3.1 through
150.1.3.254, so the packet's destination address matches this entry. Just like
before, the routing table entry tells R1 to forward the packet out its Ethernet2
interface, to R2 next. The only difference this time is that R2's IP address is
different (150.1.2.2).
Finally, when R2 receives the frame, it does the usual error
check and extracts the IP packet. R2 then compares the destination IP address to
its own routing table and finds an entry for 150.1.3.0. In this case, there's no
next-hop router listed, so R2 can forward the packet directly to the web server,
just like before.
In short, whether or not an internetwork uses subnetting, the
basic routing logic remains the same.
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