4-2 CHAPTER 4: ROUTING WITH IP
When an IP router sends a packet over multiple physical networks, it
does not know the complete path to a destination — only the next
hop. Each hop involves the following:
■The IP routing algorithm computes the next hop IP address (the next
router interface) using the routing table entries.
■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.
The Elements of IP Routing
IP Routers use the following elements to transmit packets in a
subnetted environment:
■IP addresses
■Router interfaces
■Routing tables
■Address Resolution Protocol (ARP)
IP Addresses IP addresses are 32-bit addresses composed of a network part (network
on which the host is located) and a host part (the host on that network).
See Figure 4-2. They 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
The IP address network part is assigned by a central agency, and the
host part is assigned by each network’s administrator. 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 depends
on the
class
of IP address.