3Com 2500 manual Elements of IP, Routing

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	IP routers use the following elements to transmit packets in a
Routing	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.
	IP Address		32 bits
		network	host

The boundary between network and host parts depends on the class of IP

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).