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4-2 CHAPTER 4: ROUTING WITH IP TECHNOLOGY
When an IP router sends a packet, it does not know the complete path
to a destination — only the next hop. Each hop involves three steps:
■ The IP routing algorithm computes the next hop IP address, and next
router interface, using the routing table entries.
■ The Address Resolution Protocol (ARP) translates the next hop IP
address into a physical MAC address.
■ The router sends the packet over the network to the next hop.
These routing elements are described in more detail in the following
section.
Elements of IP
Routing
IP routers use the following elements to transmit packets in a
subnetworking environment:
■ IP addresses
■ Router interfaces
■ Routing tables
■ Address Resolution Protocol (ARP)
IP Addresses IP addresses are 32-bit addresses composed of a network part (the
address of the network on which the host is located) and a host part
(the address of the host on that network). See Figure 4-2. IP addresses
differ from Ethernet and FDDI MAC addresses, which are unique
hardware-configured 48-bit addresses.
Figure 4-2 IP Address: Network Part and Host Part
A central agency assigns the network part of the IP address, and the
network administrator assigns the host part. All devices connected to
the same network share the same IP address prefix (the network part of
the address).
IP Address
network host
32 bits
The boundary between
network and host parts
depends on the
class
of IP