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Rules for Grouping IP Addresses

The original specifications for TCP/IP grouped IP addresses into sets of consecutive
addresses called IP networks. The addresses in a single IP network have the same numeric
value in the first part of all addresses in the network. Figure 4-5 shows a simple internetwork
that has three separate IP networks.
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Figure 4-5 Sample TCP/IP Internetwork Using IPv4 Network Numbers
The figure lists a network identifier (network ID) for each network, as well as a text description
of the DDN values in each network. For example, the hosts in the Ethernet LAN on
the far left use IP addresses that begin with a first octet of 8; the network ID happens to be
8.0.0.0. As another example, the serial link between R1 and R2 consists of only two interfaces—
a serial interface on each router—and uses an IP address that begins with the three
octets 199.1.1.

Figure 4-5 also provides a good figure with which to discuss two important facts about how
IPv4 groups IP addresses:
■ All IP addresses in the same group must not be separated from each other by a router.
■ IP addresses separated from each other by a router must be in different groups.

Take the first of the two rules, and look at hosts A and B on the left. Hosts A and B are in
the same IP network and have IP addresses that begin with 8. Per the first rule, hosts A and
B cannot be separated from each other by a router (and they are indeed not separated from
each other by a router).

Next, take the second of the two rules and add host C to the discussion. Host C is separated
from host A by at least one router, so host C cannot be in the same IP network as host A.
Host C’s address cannot begin with 8.

NOTE This example assumes the use of IP networks only, and no subnets, simply because
the discussion has not yet dealt with the details of subnetting.

As mentioned earlier in this chapter, IP address grouping behaves similarly to ZIP codes.
Everyone in my ZIP code lives in a little town in Ohio. If some addresses in my ZIP code
were in California, some mail might be delivered to the wrong local post office, because the
postal service delivers the letters based on the postal (ZIP) codes. The post system relies on
all addresses in one postal code being near to each other.

Likewise, IP routing relies on all addresses in one IP network or IP subnet being in the same
location, specifically on a single instance of a LAN or WAN data link. Otherwise, the routers
might deliver IP packets to the wrong locations.

For any TCP/IP internetwork, each LAN and WAN link will use either an IP network or an
IP subnet. Next, this chapter looks more closely at the concepts behind IP networks, followed
by IP subnets.

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