System information
Troubleshooting TCP/IP 7-95
Internet Routing
Figure 7-5 A Sample Subnet Mask
On some media (such as IEEE 802 LANs), media addresses and IP addresses are dynamically
discovered through the use of two other members of the Internet Protocol suite: the Address
Resolution Protocol (ARP) and the Reverse Address Resolution Protocol (RARP). ARP uses
broadcast messages to determine the hardware Media Access Control (MAC)–layer address
corresponding to a particular internetwork address. ARP is sufficiently generic to allow use of IP
with virtually any type of underlying media-access mechanism. RARP uses broadcast messages to
determine the Internet address associated with a particular hardware address. RARP is particularly
important to diskless nodes, which may not know their internetwork address when they boot.
Internet Routing
Routing devices in the Internet have traditionally been called gateways—an unfortunate term
because elsewhere in the industry, the term gateway applies to a device with somewhat different
functionality. Gateways (which we will call routers from this point on) within the Internet are
organized hierarchically. Some routers are used to move information through one particular group
of networks under the same administrative authority and control (such an entity is called an
autonomous system). Routers used for information exchange within autonomous systems are called
interior routers, and they use a variety of interior gateway protocols (IGPs) to accomplish this
purpose. Routers that move information between autonomous systems are called exterior routers,
and they use an exterior gateway protocol for this purpose. The Internet architecture is shown in
Figure 7-6.
Figure 7-6 The Internet Architecture
0
Class A
address
0 1 0 0 0 1 0 . 0 . . 0 . 34.0.0.0. 0 .
Subnet mask,
8 subnet bits
. 1 .. 1 . . 0 . 255.255.0.0. 0 .
0
Class A
address
0 1 0 0 0 1 0 . 0 . . 0 . 34.0.0.0. 0 .
Subnet mask,
16 subnet bits
. 1 .. 1 . . 1 . 255.255.255.0. 0 .
Autonomous system
I
E
Autonomous system
E = exterior gateway
I = interior gateway
I
E