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IP Addressing and How Addressing Helps IP Routing

IP defines network layer addresses that identify any host or router interface that connects
to a TCP/IP network. The idea basically works like a postal address: Any interface that
expects to receive IP packets needs an IP address, just like you need a postal address before
receiving mail from the postal service.

TCP/IP groups IP addresses together so that IP addresses used on the same physical
network are part of the same group. IP calls these address groups an IP network or an IP
subnet. Using that same postal service analogy, each IP network and IP subnet works like
a postal code (or in the United States, a ZIP code). All nearby postal addresses are in the
same postal code (ZIP code), while all nearby IP addresses must be in the same IP network
or IP subnet.

NOTE IP defines the word network to mean a very specific concept. To avoid confusion
when writing about IP addressing, this book (and others) often avoids using the term network
for other uses. In particular, this book uses the term internetwork to refer more generally
to a network made up of routers, switches, cables, and other equipment.

IP defines specific rules about which IP address should be in the same IP network or IP subnet.
Numerically, the addresses in the same group have the same value in the first part of the
addresses. For example , Figures 4-1 and 4-2 could have used the following conventions:
■ Hosts on the top Ethernet: Addresses start with 10
■ Hosts on the R1-R2 serial link: Addresses start with 168.10
■ Hosts on the R2-R3 EoMPLS link: Addresses start with 168.11
■ Hosts on the bottom Ethernet: Addresses start with 168.1

It’s similar to the USPS ZIP code system and how it requires local governments to assign
addresses to new buildings. It would be ridiculous to have two houses next door to each
other, whose addresses had different ZIP codes. Similarly, it would be silly to have people
who live on opposite sides of the country to have addresses with the same ZIP code.

Similarly, to make routing more efficient, network layer protocols group addresses, both by
their location and by the actual address values. A router can list one routing table entry for
each IP network or subnet, instead of one entry for every single IP address.

The routing process also makes use of the IPv4 header, as shown in Figure 4-3. The header
lists a 32-bit source IP address, as well as a 32-bit destination IP address. The header of
course has other fields, a few of which matter for other discussions in this book. The book
will refer to this figure as needed, but otherwise, be aware of the 20-byte IP header and the
existence of the source and destination IP address fields.
IP Addressing and How Addressing Helps IP Routing
Figure 4-3 IPv4 Header, Organized as Four Bytes Wide for a Total of 20 Bytes