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(Once the Internet address, subnet addresses, and subnet masks are in place, infor-
mation can be sent, or routed, to any destination in the Internet. Both hos ts and gate-
ways participate in the routing process.
If information is sent between two devices on a single physical network, or subnet, no
gateways are involved. The sending device encapsulates the datagram in a physical
frame, binds the destination internet address to a physical hardware address, and
sends the resulting frame directly to the destination.
If the information is sent between two devices on two different physical networks, how-
ever, the internet gateways form a cooperative, interconnected structure. Datagrams
pass from gateway to gateway until they reach a gateway that can deliver them directly
to a device on that physical network.
464
TCP/IP defines a means for the network administrator to configure the network that
best suits the network environment at a site. One method is to define the 32-bit TCP/
IP network address in terms of the number of networks and the number of hosts at the
site. The network address is partitioned into a network par t and a host part.
There are 3 common partitions—Class A, Class B, and Class C. The classification is
done by the highest order bits. For example, a 0 in the highest order bit of the 32-bit
address indicates that the address is Class A. Thus, the next 7 bits constitute the net-
work address, and the last 24 bits form the host address. The high order bits also con-
stitute the network mask for the Class A address.
The following table identifies the differences between Class A, B, and C addresses.
Class Uses Highest
Order Bits
(Binary)
Network
Address
Size
Host
AddressSiz
e
Network Mask
(Hex)
A Sites with few
networks and many
hosts
0 7 bits 24 bits 0xFF000000