3Com CoreBuilder 2500, User Manual

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4-2 C
HAPTER
4: R
OUTING WITH
IP T
ECHNOLOGY
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:
1 The IP routing algorithm computes the next hop IP address and the next
router interface, using routing table entries.
2 The Address Resolution Protocol (ARP) translates the next hop IP address
into a physical MAC address.
3 The router sends the packet over the network across the next hop.
Elements of IP
Routing
IP routers use the following elements to transmit packets:
■
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 where the host is located) and a host part (the
address of the host on that network). See Figure 4-2.
Figure 4-2 IP Address: Network Part and Host Part
IP addresses differ from Ethernet and FDDI MAC addresses, which are
unique hardware-configured 48-bit addresses. A central agency assigns
the network part of the IP address, and you assign the host part. All
devices connected to the same network share the same network part
(also called the
prefix ).
IP Address
network host
32 bits
The boundary between network
and host parts depends on the
class of IP network.